We would close our signup after a specific number. What it should be?

It depends how you are defining “performing for early adopters”. If it’s about the value they are getting and your purpose is to find if there is something you missed building or something is not delivering value as intended, then it would be a smaller number like 20 maybe (but depends on what kind of product it is). If it’s about technical performance, then it would depend on the tech scale up matrix you had created and whether actual results are matching or not — you might not need to close sign ups or might need to close signups based on your observed thresholds. In either case, I am assuming that the product is well tested before release