Hypothetical Question on a potential flop raise

[meanjean]

Your last to act in an unraised pot. Three people see the flop: say BB, random MP and you. Flop. BB bets, MP calls, and lets say you have bottom pair. Lets also assume that little coordination on the board has probably didn't help your opponents as that it would have to help you for my question. So you have bottom pair which gives you 5 outs. How many more outs would you want before you raised this pot? Is a backdoor flush (using two of your cards)enough or would you need a backdoor straight too (again using two of your cards)? Or maybe you never raise here. Lets also assume that the opponents are rather passive.

Also would this change if a 4th person was in the mix?