Hi I have a Garmin Oregon. This device can handle all maps as long as the extension ends on .img
Some other Garmins require only one filename: gmapsupp.img.
So if you load from Mapsource to Garmin the file will be called gmapsupp.img. If such a file (with another map) already exists on your Garmin, it will be over written.
I deal with this by first renaming my file on the Garmin in eg. "mapofitaly.img" before sending the 'new' file from Mapsource.
If you do this consequently you built up a nice collection of maps ready to be used on your Garmin device.
You need quite some storage space. My one has an extra micro SD card in it (16Gb).