OneDrive for Business - files in folder limited ? mentioned the 300,000 and also the 5,000 per-folder limit which some people seem to hit. In Failed Backup with id: 3 (Missing Files), switching to OneDrive v2 did successfully workaround it, but I think I’ve also seen some reports where the Microsoft limit stayed there.
For a 2 TB backup, your database will have to track 20 million blocks, so unless you prefer deduplication effectiveness over database speed (especially felt if the database gets lost), bigger –blocksize may help.