Hi tico_x00,

Post moved in appropriate thread

As your device is trainer-navigator, I hope you will not walk (run) or ride your bicycle so far away that you will need maps that will not fit on 4 GB.
Yes, you can put multiple maps on SD card of any Garmin device, but best way is not to put them as multiple files, but combined with MapSorce and transferred as one file to device (SD card), because many devices (not all) supports multiple files on device but only one file per SD card. I'm not familiar with your device, so I'm not sure.
As I can see, you're from Slovenia. Complete CN Europe + complete Adria Route 3.00 is about 2 GB, complete Adria Topo 2.2x is 395 MB, you can add to this Austria topo and some other countries topo maps and you will not exceed 4 GB.
I don't see any reason why you would need SD card larger than 4 GB.