I've been trying to import thousands upon thousands of entries into a few different channels.
Before I get too far into my current project, I just wanted to clarify that there is no limit as to how high the entry IDs can go in ExpressionEngine.
Entry IDs are stored as an unsigned integer field in MySQL, meaning the maximum value is 4294967295
More a general MySQL tip: if you need to test the import and re-import a couple of times, keep an eye on the auto_increment value for the entry_id column of exp_channel_titles. Either reset auto_increment after deleting a test import, or set the entry_id value as well on import.