In Gwent, it is almost always correct to play decks of the smallest possible size - 25 cards. There are a plethora of reasons for this, but they all revolve around improving the consistency of your deck, allowing for your game plan to be executed more reliably. This is ever more important in Gwent (compared to other CCGs) because of the limits placed on the amount of Gold and Silver cards that can be included in any deck. Having a larger deck size would result in seeing these more powerful cards less frequently, and would decrease the average value of cards you play.
Increasing your deck's consistency through thinning
The concept of thinning encompasses cards that remove other cards from the deck in some way and thus decrease the remaining deck size. Thinning can take all manner of forms, from units that pull out copies of themselves (known as 'mustering'), to units that allow you to play a specific card from your deck, to units with the words 'draw a card' imprinted upon them.
The most important component of thinning is the removal of bronzes from the deck. This helps optimise your draws at the start of rounds 2 and 3, maximising the chances to grab your powerful silvers and golds, and making your deck more consistent. Overall, Nilfgaard and Northern Realms are the factions with the most thinning options, but there are plenty in every faction (as well as among neutral cards), so pretty much any deck can be built to thin as effectively as is required for your game plan.
A note on thinning and sequencing
It is desirable to increase the average card quality and power of the cards you’re playing by thinning as many bronzes out of your deck as you can before using draw effects like Avallac'h, or effects that allow you to play one of the top cards of your deck, like The Last Wish.
For the same reason, it is also better to thin early in the game, in order to make your later round draws and mulligans better. It is also vital to limit the impact of randomness by making sure that every card you can get from a random effect would be useful. For example, you want to have a resurrect target in your graveyard before using First Light’s rally option to potentially pull a Priestess of Freya. But all of this depends on what else is happening in the game, which makes sequencing a very complex topic that perhaps deserves an article of its own.
The costs of thinning
Since thinning is generally seen as a benefit, cards that thin have one or more drawbacks to balance that out. These include:
Restricted mulligan options
Examples: Clan Drummond Shieldmaiden, Blue Mountain Commando, Foglet, Roach.
You are often obliged to mulligan these cards away, which constrains your ability to freely optimise your hand and blacklist. Furthermore, too many of these effects in a single deck can lead to hands that cannot be fully optimised with 3 mulligans, which decreases both the power and the thinning provided by these cards considerably.
Decreased power or randomness
Examples: Emissary, Elven Mercenary, First Light, Barclay Els.
There are definitely ways to mitigate both of these drawbacks, for instance by playing multiple Impera Brigades with your Emissary cards or making sure that every bronze unit in your deck will provide good value in the event that it gets pulled by Rally.
The opportunity cost of using silver and gold slots for thinning
Examples: Nature's Gift, Royal Decree, Alzur's Double–Cross, Ge'els, The Crones.
This drawback is somewhat counteracted by the fact that these cards provide another benefit in addition to thinning your deck - either adding power to the cards they pull (like Alzur's Double–Cross) or giving you a good deal of choice (like Nature's Gift).
Lack of Flexibility
Examples: Reaver Scout, Imperial Golem, Clan Dimun Pirate, Eredin.
Some thinning cards, such as Reaver Scout are conditional on what cards you have on the board, or in your deck. Other cards, such as Clan Dimun Pirates and many of the muster units, often oblige you to play them in round 1, or risk drawing extra copies in later rounds.
The perils of thinning too much
While the previous section considered the costs associated with individual thinning cards (or card sets), there are also drawbacks to running too many of these cards in a single deck.
Everyone starts a game of Gwent with 10 cards, and draws 3 more over its duration. Thus, in a typical 25 card deck (which is what you'll be running under most circumstances), it is imperative that you include 12 or less sources of thinning, otherwise you'll simply run out of cards! (there are some interesting exceptions to this - discussed in the next section)
There are definitely diminishing returns to thinning, exacerbated by the mulligans at the start of rounds 2 and 3. Thinning 10-12 cards decreases the value of your round 2 and 3 mulligans and thus also reduces the value of situational cards like Dimeritium Bomb and Mardroeme. If you thin the full 12 you are obliged to play every card in your deck, no matter how bad it might be in any given matchup. You might also occasionally run out of targets for targets in your deck for cards like Aretuza Adept, though this problem is lessened by the ability to mulligan them back in if the need arises, so it's rarely a huge issue.
Another knock against excessive thinning is that there are a number of cards in the game that allow your opponent to thin your deck, which can potentially leave you with no cards to draw. A perfect example is Tibor Eggebracht, a common sight in modern Nilfgaard decks. You really want to have a bronze left in your deck when he stampedes onto the battlefield, otherwise your opponent is getting away with playing a drawback-less 23 point gold! When meta decks start to thin a bit too much, players may look to punish this by running more of these types of cards. A full on 'mill' Nilfgaard deck is also a real thing, the main goal of which is to run it’s opponent out of cards using cards like Avallac'h, Sweers and Albrich.
Using thinning to your advantage
Despite all the pitfalls of excessive thinning, there are several ways to benefit from it, or at least mitigate the problems outlined above.
This refers to cards that can thin your deck, but don't have to, and are still powerful if you don't. There's only a few examples of these, most notably Vicovaro Novice choosing to copy the ability of an Ambassador rather than an Emissary. But if you're in a position where you're bleeding your opponent out (usually in round 2 after winning round 1), you can stop several more of your cards from thinning your deck - for instance, using clear skies instead of rally from your First Light.
Including more than 25 cards
The more cards you run, the fewer silvers and golds will find their way into your opening hand. Ordinarily there are other drawbacks too, such as decreased consistency and having to pay a cost for including more bronze thinning, so this is almost never correct. Unless, of course, the thinning effects you can run are so good that the (marginal) benefit of adding another one to your deck outweighs the costs outlined above without considering the benefit of thinning (since by adding an extra card to your deck, most of that benefit is actually lost). This doesn't happen often, but Nilfgaard decks can sometimes justify running more than 25 cards to utilise Imperial Golem, Emissary and Vicovaro Novice to their full potential.
Thinning in reverse
And finally, there's a few cards in Gwent that allow you to increase your deck size by putting cards back into your deck. The most notable examples are Ciri: Dash, Assire var Anahid and Nenneke, and these can be used to enable excessive thinning and/or to provide a payoff for it. For example, you can shuffle back a roach and/or other powerful silvers with Assire to add even more power to your final round gold and increase the card quality for your round 3 draw + mulligan.
All good thins must come to an end
So there you have it - thinning is a vital tool that will help you succeed, and comes with a plethora of benefits and drawbacks. But thinning is just one piece of the Gwent puzzle, rather than an ultimate end goal. So thin well, but thin wisely - Gwent isn’t always a thinning game, but it is always a thinking one!
This is a pilot for a potential series about Gwent concepts. Let me know whether you like the format and content as well as what other topics you'd like to see covered! You can find me in the comments below or on reddit.