It depends on device
here
[Only registered and activated users can see links. ]
you will find links to these threads:
Updating GarminAsus M10 to use map more than 2 GB.
How to view hidden map folder in 1xxx nuvi series
Garmin Nuvi MapSet format- how to store them in your device
Multiple maps on SD Card
Multiple maps on nuvi 1XXX series
Multi maps on Nuvi 7x0/7x5
Oregon Maps
Multiple maps on 3790 SD Card post 18