There are two absolute limits that affect Mobile PC.
1. You can only include 2025 tiles ("maps") in a single mapset file on any Garmin device, including Mobile PC.
2. The mapset file must be under 2GB in size in Mobile PC.
Terry and I have just (last night) done some testing and have demonstrated that Mobile PC will not use a file that is larger than 2GB. The map tile limit (2025) in all Garmin devices is well documented.
Mapsource will stop you if you try to make the file more than 4GB but it will not stop you from selecting too many map tiles to include in one mapset as long as the resulting mapset file won't be larger than 4GB (I just tried it and I was over 4,300 map tiles without any warning).
When you are using the map selection tool in Mapsource, if you click on the MAPS tab you will see the list that you have already selected. At the bottom of that display you will see the number of map tiles selected and the projected file size. Keep those within the limits for Mobile PC and you'll be okay.
...ken...