A BR question

[Baby Mantis]

Okay,
So I know for NL you want to buy-in for the maximum amount you can so you are not short-stacked and can maximize your profits. You also need 20 at least to 30 buy-ins for the level you are playing for good BR management.

For limit I also know that you need at least 200 and better 300 BBs for the level you are playing. Now this could of been said somewhere else and I just missed it, but how much should you sit with at a cash game short-handed or full-ring? Should it be 20, 30, or more BBs?
Thx.