Right, seems like every week there is some new post about bankroll management and questions like "how many buyins do I need for xxxx?" and the like. I checked the stickies and there doesn't seem to be anything comprehensive up there so I decided I'd write something now because I don't want to study.
An important thing to realise is that there are no set rules you can follow when deciding on bankroll management. Hopefully here I can point out some of the things that should be taken into consideration though.
The first thing you need to consider when trying to decide how big your bankroll needs to be in order to be sufficiently bankrolled to play a certain game is your skill level. The ammount of variance which a certain player will encounter when playing poker at a certain level is largely a function of his skill level relative to the other players in the game. Put simply that means that if you crush a certain game then you should, in the long run, be able to play with less variance than if you were playing in a game in which you are a breakeven player or marginal winner. Thus you can get a way with having a smaller bankroll.
Another factor to consider is the general level of aggression in the game. A very aggressive game will induce a lot of variance for its players because they are being forced to make marginal decisions for large ammounts of money (read: large percentages of their stack) very often because their opponents are putting pressure on them every chance they get. Eventually you need to take a stand against this aggression in spots where you are not 100% sure that you are winning. Sometimes you are right and sometimes you are wrong, the fact is that these types of decisions all contribute to increasing variance and mean that you should try and have a bigger bankroll before tackling these types of games.
This point also applied to short handed games like 6max and heads-up. The less players in the game the less likely it is that anyone has a premium hand. Therefore being very aggressive can often be a good strategy for winning in these games. Keep in mind though that this will push up your variance and the bankroll needed to play at a certain stake will increase the shorter the game gets.
Also a note on the aggression paragraph. I feel that it can be difficult for low stakes player to get a good idea of what a truely aggressive game is. I often hear people saying that you will have higher variance if you play against really bad players, suce as those found in the SE or the Fitz. This is a complete fallacy. Bad players are easy to play against, you make a good hand, you bet, they call and you take their money. Yes they will suck out more often then good players, that is true. But that is only because they get their money in behind more often then good players and the suckouts make it appear like a high variance game.
So the best advice I can give when trying to decide if you are bankrolled for a game is to consider the above factors. As a general guideline, if you are playing in a low aggression live game that you beat for a high rate then you can get away with a smaller bankroll like 15-20 buyins. If you are not so sure of your abilities then try and wait until you have a larger roll before stepping into the game, more like 20-25 buyins. In an agressive high stakes online game then these numbers need to be increased to account for the different game conditions. Personally I won't play 5/10 NLHE online without at least 50 buyins in my bankroll and I'd even consider that marginal enough. Why? Because it's a highly aggressive game in which I am not certain that I'm a winner.
relative skill level up, bankroll needed down.
aggression up, bankroll needed up.
easy games: 15-25 buyins.
meadium games: 25-35 buyins.
hard games: >50 buyins.