Here's my rule for numbers.
100 target customers that I've done customer discovery interviews with.
10 customers (who've paid / given me money)
25 who are active beta testers.

I usually use that order of things as well. I like to get early adopters to give me money to validate their feedback, then 25 to give product feedback to test usage & retention, but in parallel I talk to as many potential customers as possible to learn.