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.
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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.