You should buy 1 licence for every active server that runs DeviceAtlas. This means that if you have a load-balancer with, say, 2 active servers answering requests, you should buy 2 licences. If you have a master-slave cluster with, again, 2 servers (1 master and 1 slave), then you may buy only 1 licence as DeviceAtlas will only be active on 1 server at a time.