Suppose an enormous database. For instance I've a lot more than 4294967295(for mysql INT) customers. In almost any column which sort is int and auto_increment, what goes on in the event that column achieve the max value?
The int will overflow and you'll not have the ability to place more records to that particular table.
You receive the mistake
#1062 - Duplicate entry '4294967295' for key 'PRIMARY' upon the following place.
An overflow will occur and what goes on next is database dependent.
Although, are you certain you will have 4 billion records inside your database? Consider it. Theoretical limits will not necessary impose problems within the real life.
If you are concerned about that, opt for a UUID.
Error will occur. You are able to opt for bigint (64 bits), that's always much better than UUID that's 128b (if you do not require it).