What if django model incremental pk finished
The database will raise an error and inserting whatever you're inserting will fail.
If you are using MySQL, the error will probably be something like this : ERROR 1467 (HY000): Failed to read auto-increment value from storage engine. 
The limit for django's auto increment id is  2,147,483,647. So it's pretty unlikely that you will hit this limit unless you're storing extremely large amounts of data. If you are using large amounts of data, you can use the bigint type for the field instead.
In Django, if you don't specify primary key, it will add an integer autofield. My question is that if this incremental id, reaches integer limit, what is expected to happen after?