re: Poker & Playing out of bankroll
Okay, yeah if you don't really take the game that seriously, or you're financing your poker play through other means, then sure who needs BRM? Who cares if you're a winner at lower stakes yet when you have $1k in your pocket and this $5/$10 action is so juiccccccy! If instead you keep your roll separate from other money, are looking to improve your game, move up in stakes, and manage your risk, then you need to utilize BRM.
You absolutely must have a MINIMUM of 20 buy-ins. This figure has been shown to provide decently sufficient protection against losing your bankroll due to variance. Plenty of players, myself included, prefer a larger number. The one caveat I'll make here are folks looking to start playing $1/$2 or whatever your lowest stakes are at your casino.
If for whatever reason you can't scratch together the $4k to be minimally rolled for $200nl then first you have to ask, "Should I really be putting this money I have on the felt?" If the answer is "Yes" or, "Shut up Mase" then let's put together what you can and start taking shots. I'll further suggest you take whatever you have to play with into 3+ bullets, yes even if that means short-stacking. People just don't play optimally when they know they're on a single buy in.
But don't you touch that $2/$5 until you have a minimum of $10,000 in your roll, I'm dead serious! Maybe you're thinking, "Whatever, I have $4,000 I can easily handle a $500 buy in".
But here's the thing, let's say you do sit, and on top of that stack that donkey who had $500 in front of him. Now you have $1,000 in your stack and life is great! Except your entire roll at this point is $4,500 and $1,000 of it is on the table right now. Are you really going to play optimally when you're facing a decision to put 22% of your entire roll in the middle? Here's a hint, you won't.
Ultimately BRM is about two things:
1) Minimize risk of ruin
2) Ensure bankroll will not affect poker decisions
#1 is why 20 buy ins is the minimum. This is required, in order to mathematically manage your risk of ruin.
Ultimately, the actual size of your roll should be whatever amount of buy ins provides psychological protection from variance. Those playing with too few BI's in their roll will tend to be weak-tight. They avoid +EV situations when variance is higher because they are worried about losing the pot. Conversely, a player properly rolled can sit down and simply focus on making +EV plays that present themselves, a much more beneficial position to be playing from.
So that wall of text is the long version of the answer to your question, "How many buy ins do I need?"
At least 20, probably more