Do you have to have exactly 60 cards in a Magic the Gathering deck? Some people I've talked to at FNM say yes, others say no.
No, not at all. You probably should keep it near the minimum since a deck of minimum size will provide more consistent results than a larger deck, but you are not obligated to do so.
- For constructed events, your deck must have at least 60 cards[CR 100.2a] (plus an optional sideboard of no more than 15 cards[CR 100.4a]).
- For limited (draft and sealed) events, your deck must have at least 40 cards[CR 100.2b].
Additionally all decks no matter the format must be "able to be shuffled within a reasonable time"
There are many formats that impose additional limitations. Notably,
- For Commander games, your deck must have exactly 100 cards (including your Commander)[CR 903.5a].
I could quote you the rules, but Battle of Wits is much more convincing :)
Note: When this answer was originally written the side board was limited to exactly 15 cards. This has changed. I have updated this information to match current rules since it doesn't affect the answer.
You certainly don't, not in any normal MtG format. 60 card decks are the minimum in most Constructed formats, but there is no upper bound. Commander decks are all the same size... but they are 100 cards big, not 60. In typical Limited environments (Draft/Sealed) the minimum size is 40, with an upper limit determined only by the size of your cardpool (and even then you can add a LOT of basic lands on top).
If you want to prove to someone that a Magic deck doesn't have to have exactly 60 cards in it, the quickest way is probably to refer them to Battle of Wits - a famous card that doesn't even make sense in a world of uniform 60-card decks.
By default, sixty is just the minimum in constructed, and the minimum is forty in limited:
100.2a In constructed play (a way of playing in which each player creates his or her own deck ahead of time), each deck must contain at least sixty cards. […]
100.2b In limited play (a way of playing in which each player gets the same quantity of unopened Magic product such as booster packs and creates his or her own deck using only this product and basic land cards), each deck must contain at least forty cards. […]
100.5. There is no maximum deck size.
The only official variant that changes this is Commander, which requires exactly one hundred cards:
903.5a Each deck must contain exactly 100 cards, including its commander.
For the record, I have not played in a few years. If you are playing with friends than anything can go, if they are ok for it.
Tournament rules are still as follows, "Players may have a sideboard of up to a maximum of 15 cards, and exchanges of cards between games are not required to be on a one-for-one basis, so long as the player adheres to the 60 card minimum deck size"
There was a rule were you could only have 4 cards with the same name unless the card had special rules limiting it, at the time you could only have one "Wrath of God" or any non-named land for example. Having said that most people would run around 15 land and around 45 other cards... in this case do 16 land and 44 other cards leaving you only 11 different cards give or take. You would do this to increase your odds of getting your strategic cards. When drawing randomly, it's not luck it is a number game, play the odds!
Having said that, I once had a massive 120 card deck. It played awful due to the odds being against me with every draw.