I would say at least 25. A standard 5x5 card has 24 prompts and a center that's either Free Space or Wild Card. So you need a minimum of 24 to fill that card without repeats, and the extra means people can put a regular prompt in the center if they wish.

We have some huge lists with 80+ prompts, but most are probably between 25-50. Browse around the lists on the Bingo Card Generator and you'll see the range.

If you're thinking about hosting a fest, please do! It's not hard, it's fun, and we love seeing whole new themes even though we have some old favorites too.