How many beacons can be placed on a map?
 

I set four of them on NWE map, but can see in game only three. More in detail, I set four beacons, one for each available air base in Blue.mis file. Starting a new DGen career in that map, I got an air base having its own beacon, and switching by proper command, I get a total of three NDB: AM, AR and BD. I can see them marked on minimap too. Can't see the fourth one.

Thank you for help,
GB