There of course is a limit but when i tried researching this i found nothing.

I did a import a few days ago to do a mass update on 7.7mil products. i did a 1000 at a time i think possibly higher.

I must admit they did not contain alot of information 2 rows doing one row the the other. but it seemed to run fine. You can possibly chuck alot at it but you just have to be realistic in the end. With my imports i do chunks of 100 too for performance it works perfectly.

If you really want to test it, you will just have to keep putting tests through and see when it chokes.