Memory estimation for cache is computed with the number of redirection requests

as I see, redirection requests consist of some URL duplicated among the redirection. so It wouldn’t be the number of URLs in our database.

If what I think is right, we have to calculate the memory size with the number of shortening requests. because the number of shortening requests is stored in a database.

please, take a look at my opinion

Course: Grokking Modern System Design Interview for Engineers & Managers - Learn Interactively
Lesson: Requirements of TinyURL's Design