I tend to be more of a pantser, but I do need to know a few basic things to start with. I need to know the main character(s), their motivation, their goals. I need an idea of the setting, the lore, the history. I have to know what the general plot of the story is going to be—what the main character(s) ultimately seeks to/must accomplish. Once I feel like I have enough to get it going, I can come up with ideas as I go, whether spur-of-the-moment or later things figured out in advance. The first draft isn't going to be perfect, but I can sort out the writing and plot details on subsequent drafts.

So the answer for me is roughly "enough that I don't feel like I'll hit a dead-end".