Generating unique username based on input?

Question

Database

Hello. There is a task: to give the user a similar list with possible usernames, if his input (in the field) is already in the database.
Example:
new user Vasya enters the name "Vasya", but it is already taken. The system offers him, for example, such a list: "Vasya1", "Vasya123", etc.
So, gentlemen, what can you advise from the point of view of implementation or theory? (What to read, what to watch). Thank you for your attention.