Google sheet login with restrictions

Hello I wanted to put restriction for number of login from my app I am using Google sheet as database.

How would you uniquely ID your users?

By email address?

At setup time for each new user, add a row to your remaining uses sheet with the starting count for that user. (2 columns)

At app start, grab the user's count.

When it arrives, decrement it and rewrite it to the row it came from.