I think this answer is very strange and means that there is an inverse relation between number of URLs in the db and the probability of collision, and based on this equation, the probability of collision is 100% if there is just one record in the db!

Course: Grokking Modern System Design Interview for Engineers & Managers - Learn Interactively

Lesson: Design and Deployment of TinyURL - Grokking Modern System Design Interview for Engineers & Managers