Limit - 300BB
NL - 20 buyins
These seem to be pretty much the accepted figures for the above.
Tourney - 100 buyins (or less - do ya feel lucky?).
SnGs - 40 buyins? Maybe less, maybe more.
All depends on how many players are involved and whether you are prepared to drop down limits when your bankroll shrinks.
I have an immensely convoluted bankroll "system" for multitabling which also includes hands played and blinds won, and I"m very happy with it - I have to prove I"m winning before I move up, and gearing down can be done swiftly. Hopefully I"ll never go broke. Whatever you decide to use, the tricky part is sticking to it, especially when the downturn comes - and remember that, for a losing player, no bankroll is big enough...