lol come on dude, how can you expect an answer that's remotely accurate with that vague of a question? If you're a ninja you could get away with 20 bi. If you're not a ninja you're gonna want like 50-100. What's the rake like on the site you're playing on? Are you bumhunting or playing regs? All of these things are going to dictate your winrate, which is the main decider in how many buy-ins you should have.
Edit: Here's a tool that can give you a general sense of how many BI you will need. http://www.evplusplus.com/poker_tools/risk_of_ruin/
I'm pretty sure the standard deviation will be a little higher than it is for 6max and FR + it also varies from person to person. I'd put it anywhere from 100-150 but I'm not entirely sure.
I apologise for the vagueness of the question.. I just assumed it was a standard figure. Thankyou for the link it was very useful.
I am playing on pokerstars and want to ensure I don't go busto and am currently learning heads up play. I intend to play weaker players (ie not regs) and will be full stacking.
thankyou for the reply.