If you disable all the maps in the Nuvi's map settings, you will be looking at the basemap. The basemap is stored in the file \Garmin\gmapbmap.img on the internal drive. You have an older model with a more simplistic basemap, but starting with the next models after the 700 series, the basemaps contain routable roads and DEM (Digital Elevation Model) data that produces shaded terrain. The terrain data is very low resolution however, so it only display when you zoom way out to the 20+ mile scale IIRC. Nevertheless, I don't think your 750 has this feature.
You can install one additional map in internal memory, and a second one on your data card. The files will each be named gmapsupp.img. Starting with the 1200/1300/1400/1600 series, Garmin did away with map filename restrictions, so you can add as many map files as you want on these Nuvi's and call them anything you like.
On your unit, you can still install multiple maps. Get all your maps into Mapsource, then choose the areas you want from the first map using the selection tool. Now use the dropdown menu in the upper lefthand corner to choose another mapset and select the parts of it that you want. Repeat this as needed for all the maps you want to load. In the Maps pane on the left, you will see all the different segments of the maps you have selected this way.
Connect your nuvi and choose Transfer > Send to Device. Make sure that Maps are checked. This will transfer all the different maps into a single file on the Nuvi named gmapsupp.img. On the Nuvi, you will be able to turn on and off the different maps using Menu > Tools > Settings > Map > Map Info.
Beyond this, there is another limitation to the amount of map data you can install. The nuvi can only access 4,000 individual map tiles. So it's possible that you will exceed this limit before you exceed the capacity of your storage card. Map tile size is determined by the mapmaker, so there is no "rule of thumb" as to how large an area can be covered by a given number of tiles.