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Assuming by ‘map’ you mean ‘collection of individual map tiles covering a particular area’, then it is possible using mkgmap to a) create a gmapsupp.img containing a single ‘map’ with multiple tiles and b) create a gmapsupp.img containing multiple ‘maps’ with multiple tiles.
Using mkgmap, you can create multiple tiles for the various areas you want, then combine them into a single gmapsupp.img. Each tile will be deselectable individually, which may be all you need. Alternatively, if you want to be able enable/disable entire countries/regions, you will need to create separate mapsets (with different family IDs) for each region then combine these using mkgmap into a single gmapsupp.img