As with most things Garmin, it depends on Garmin imposed limitations, not on the available storage, for maps (internal and/or MicroSD card size):
There are two limits that are imposed by Garmin (both in their device firmware and their file system), these are:
- A single map file can only be a maximum of 4GB in size; larger map files (over 4GB) will not be recognised and won't work.
- The total number of map tiles cannot exceed the total map tile limit (across all maps installed); if they do, maps will not show up in the map list on a device, and you may experience other map rendering and navigational issues. This limit is NOT dependant on the maps being enabled; even disabled maps are included in the tile limit calculation. The exact map tile limit depends on the Garmin device, and Garmin do not publish this limit.
The number of maps that can be installed on a device is not the limiting factor (apart from storage space); the limit will be the total number of map tiles across ALL the installed maps on the device, or a MicroSD card (or both, where applicable).
These limits are the same regardless of where the maps come from; so Garmin's own "official" maps have the same limitations as any third party maps.
If you exceed the Garmin, map tile limit and experience the issues described above, then I would strongly recommend that you only have maps on the card, or the devices internal storage (or a mixture of both) that you are planning to use at the relevant time; remove all others (or at least those that you do not plan to use shortly and store them on your computer).