“Design partner” is a startup term of art for customers that aren’t really customers yet.
In the early phases of a company, you're just looking for someone, anyone, to use the beta software, give feedback, and confirm you're on the right track. Monetization isn’t the goal – yet – and so these “free” design partners get early access in exchange for feedback – i.e., commitment engineering.
One question startup founders will deal with is “how many design partners should I have?”
I think the right answer is two, because it’s the minimum number that’s not one.
Too many design partners is bad, because you risk losing focus. You’re a small team, with limited resources, and can’t build something everyone will love.
One design partner is bad, because you risk over-fitting. Perhaps they have quirks or eccentricities that make them non-representative of the broader market. You could build yourself into a corner.
So I think the right number is two. It’s enough to make sure you’re not building a one-off, but few enough that you aren’t spreading yourself too thin.
If you are fortunate enough to have a big pool of interested design partners to select from, pick the two that are most similar, and will give you the best feedback. You could always do periodic “checkins” with the other interested parties, demoing the latest and keeping them warm for when you’re ready.