“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 try your janky software and tell you if you're on the right track. Monetization isn’t the goal yet, and so these “free” design partners get early access in exchange for their patience and feedback – i.e., commitment engineering.
One question startup founders will often ask is “how many design partners should I have?”
I think the right answer is two, because it’s the smallest number that isn't 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 for everyone.
One design partner is also bad, because you risk over-fitting. Your solution needs to be broadly-applicable, and perhaps this particular customer has quirks or eccentricities that make them non-representative of the rest of the market.
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.