The numbers suggested by Collin are reasonable but a bit on the conservative side except maybe for heads up SnGs. Just want to add, that with MTTs is seldom practical to play only one limit, and if you play an interval like 1-5$ or 5-25$, then its going to be your average buyin, which determine the bankroll requirement, rather than the highest buyin.
Also big field MTTs are not practical, if you are trying to build a bankroll and move up. This is because, variance is so high, you might have to play for years to even get near your true long term results. Or alternatively you get lucky and bink a big score quickly, which then cause you to move up, before you have the skills needed to beat those higher games.
So if you want to play MTTs, its much better to focus on those with small to medium sized fields like maybe 300 or less players. And for those a 500$ bankroll will be fine for a mix of 1-5$ MTTs, while a 2.500$ bankroll is needed for a mix of 5-25$ MTTs.