depends on what it is. I usually do several just to give more types of situations to respond to. there's been plenty of top levels I've skipped over because the situation presented wasn't one that fit my character at all.
Personally I prefer several short-ish prompts. By short I mean like. 3-5 sentences. But also having several prompts increases the chance that I'll find something I wanna reply to.
chaggit: i normally do around 4-6 prompts for each character but they're sometimes on the longish side. I've been considering doing shorter ones because....well...im lazy now!