How to make a system of advertisements showing evenly?

Question

Algorithms

Hello everyone, tell me, please, with the task:
How to make a system of advertisements so that they are all shown evenly?
Example: I have 5 ads, they are all currently shown randomly, which is not fair.
I want to make sure that 5 advertisements are shown approximately the same number of times. So that later, if you add 1 more advertisement to them, then it will be shown with the same frequency as it was, only already divided by 6.
(that is, making a counter that takes into account the number of impressions of the current ones - and based on it, showing the one that has the least number of impressions - is not an option, because when adding a new, 6th ad, it will be shown many times in a row until it catches up with the previous ones Resetting the counter for all ads when adding a new one is also not an option)
All ads are equal in priority to each other.
What are the ways to solve it, how to google it, where to look, and what did I forget to mention for you?
PS the situation is much more complicated, the example is simplified

Answer 1

Round-robin (algorithm)