TheRedMage says... Accepted answer #2
Well, it depends on the format.
For pretty much every format in which you are going to have a 60-cards deck (i.e. Standard, Legacy, Modern, Vintage, and Block Constructed, unless I am forgetting something) your sideboard is going to be 15 or less cards. You can put any number of cards from your sideboard into your deck, and the other way round, between rounds, provided you end up with 15 or less cards in your sideboard and 60 or more in your deck. For example, if you are playing a Battle of Wits deck with 238 cards in it, you can't sideboard into a 60-card deck after game 1, because that would result in you having a 193-card sideboard when game 2 starts; but, if you are playign your average 60-card deck with a 15-card sideboard, you can start game 2 with a 75-card deck and no cards left in your sideboard if you wish.
In Tiny Leaders, your sideboard can have up to 10 cards. This is the only constructed format I am aware of in which there is a non-15-card sideboard. Some groups play Commander with a sideboard (usually up to 10 cards) but it's my understanding that it's not the norm.
In individual Limited formats, every card that is in your pool and not in your deck is part of your sideboard. For most FNM-level events (read: everything that doesn't have you fill a decklist), limited formats also use the so-called continuous deck construction rules, meaning you can put cards from your sideboard into your deck, or the other way round, before game 1. Your sideboard is also assumed to contain an infinite number of copies of the five basic lands, meaning if you are a very indecisive player you can do what I did at the Fate Reforged prerelease, in which I was playing Jeskai for the first two rounds, then Temur with a white splash for the third and fourth, then Mardu with a blue splash for the last (gotta play that Flying Crane Technique).
BboyGeologic says... #1
Up to 15.
May 11, 2015 11:21 a.m.