Those mean that device is the 2nd or 7th device on the network to use that hostname. The addition of a number is a feature of Bonjour networking that Airplay uses to see other Airplay devices. What's causing it on your network are ghosts of your own devices not clearing themselves properly. When the device reconnects to your network or gets a new IP address, it see these ghosts and renames itself to not conflict with the ghost device. Harmless, yet annoying.