I want to collect registrations for an online-program per user and display the number of all registrations for the program on the registrations page.
Which is the best implementation for this use-case? I´m not sure how to collect the registrations from the user table (per user) and where to store these data (for all users)
Assuming you have multiple programs in a Programs sheet, you can:
Add a button in the Program details view to let users register for it. The button will open a form where you can collect the user’s email/ID, the Program ID and whatever info you need related to the registration. These will write to the Registrations table.
In the Programs table, relate the program ID column to the same column in the Registrations table, make it a multiple match and use a rollup to count the number of registrations.