Public applications that require user sign-in?

Hello all, apologies in advance for double post as I just realized I replied to a post that was in accounts, not in the questions section.

I’d like to make an app to have as many users as possible. And I would like for them all to sign-in so that I have their data to email market to them on a monthly basis along with updates. Am I correct that I’d be charged $10 per user per this link? AppSheet Pricing

Or is there a way to have unlimited amount of users at a set price?

Otherwise, I’d be limited with having a somewhat generic app that isn’t customized to the specific user by having the Publisher Pro subscription as mentioned in @Peter 's original post up above?

I was hoping there was a secure Publisher pro option.

Thanks in advance!
Sy