Just as an FYI ASyD, the rule is that the Sideboard is ALWAYS 15 cards (or 0 cards.) If you're just playing casually, many play-groups don't pay much attention to sideboarding rules (i.e. size of sideboard, when to swap cards out, etc.) or even necessarily use sideboarding during a match, but if you ever do a sanctioned event (Friday Night Magic, for example) you'll have to include exactly 15 cards in your sideboard if you have one.