I'd agree with the 20 M statement, I wouldn't think that is technically a problem. They currently market their US highway/driving maps as having between 4 and 5 million points of interest.
I'm also not sure how you see what POIs are loaded on the PC maps. On my unit, you can see a a list, the same as way points. But I have yet to view the custom POIs on the MapSource Map viewer on the PC. I never noticed if they come in via 'receive from device' the same as WayPoints do.