How many times does the migration tool have to be run?

How many times does the migration tool have to be run?

I'm testing out a migration for a test user that has 30,000+ messages. The first time it was run, 20K messages were transferred, followed by one that was ~9,000, then ~1,000 and 150 on the 4th one. How many times does the migration tool need to be run in order to insure that the migration transfers everything?

Mahalo,
Dave