The Digital Globe demo is pretty useless. The Basemap is not essential but I wouldn't delete it, it's not taking up much space and it will give you some mapping when you zoom right out, and the GPS does sometimes try to make use of the DEM information. You can turn the basemap off in the menus if you want, but there's not really any point.
You can add as many other maps as you want and have space for, from any sources that can provide maps in the gmapsupp.img format. Just rename the image files, so for example you could have portsmouth.img and france_n.img and france_s.img and india_s.img - and you can have as many as you want turned on in the menus** at one time, though if the tiles overlap it may be unpredictable as what appears on top. But even that is no problem, for example I often have Metroguide (Garmin's own mapping for UK and Western Europe, now getting a bit old) and a more recent local OSM map both switched on at the same time. Plus an overlay of contours only, sourced from another place.
Place all map files in the same directory you find the preloaded maps in - or in the equivalent directory on the SD card, which gives you loads of expansion space.
To be honest most people would just download a single big map of UK and western Europe - but there certainly is some merit in maintaining a small 'local' file with the latest updates to local footpaths etc.
** it can get a bit confusing if you have several small tiles all acquired from the same source - although they have different file names and will appear as separate menu entries, in the menu they are quite likely to all be named the same - because the map name is set up by the source software.
