Limit or no-limit?

I'm going to assume no-limit. A full buy-in is 100BBs at most sites, yes. Most people recommend buying in full. There are specific advantages and disadvantages to buying in for less than a full stack. Some sites let you buy in for more than 100BBs, which makes things interesting.

I almost always buy in for 100BBs exactly. I'm just used to how the play goes when I have that much in my stack.

For more ... search for "short stack strategy" and also look up some of excession's old posts on why he usually buys in for about half a stack.

In limit, I would not want to have fewer than 25 BBs, 40 is better.
If you can cap the nuts preflop, and post flop, that is 12BBs in one hand.

It is an unlikely scenario, but it can happen, and you want to charge the maximum when you have it.