Hello Tom,
Thank you for the response back.
1. I will use the membership plan slug. <– I really needed to know this.
2. I believe that the setting is correct.
I am testing it with a product "test-sample-ticket". My user account is part of the "family-membership-2020" plan has been able to buy more than the 4 allowed.
Should I change the settings in your point 2?
I don't want the members to be able to add more in their cart after their maximum. Am I missing something?