10-15 users would be covered in the base private pro plan price. Just in case it wasn’t clear, the first 20 users are included before you are charged for additional users.
A private pro plan will provide restricted access security to the entire app, so only you can determine who can get into an app.
You can still secure your data with any of the other plans, but it becomes more difficult for large groups of people. In this case, you would have to create a row owner column for every single user that you want to be able to see that row, and maintain the emails of every user you want to grant access. You can do this by creating a set of sequentially numbered columns in a google sheet which turns into a single array column in glide they you can assign row owners to, or you can just assign row owners to multiple columns in glide.
I want to stress that visibility and filter controls are not secure. They may hide data from view, but that data is still downloaded to a user’s device and easily accessible if you know how to get to it. Row Owners and/or a Private Pro plan are the only guaranteed ways to make sure your data is secure and only visible to those that should see it.