depends on ....
what stakes u play at..
how good u are...
how u handle variance...
everyone has different approaches - conservative or aggressive
aggro approach [some have applied even more aggro approaches, some of that i know did ok and some spewed away there rolls]
20 buy ins up to 25nl
30 buy ins from 50nl up to 100nl [depending on your style of play, how good ur]
200nl+ 50 buy ins at least... [can be less but again all dependent on style/ability and how good ur at knowing/having the discipline to drop down and grind the roll back up]
u can take shots at the next level worth 2 to 4 buy ins if u feel u are good enough or to test the waters so to speak..
eg - ur playing 25nl with a bankroll of $600, then u could take a 4 buy in shot at 50nl, if u lost that 4bi buffer then drop down to 25nl with 20 bi"s intact...