I know for my ipad(2), the (2) is because I had 2 iPads under my devices at one time. Default name is IPad so it gives it (2) to make the name unique and not clash with existing ipad name. But for these huge numbers (72)? Hmmm, I scanned the Apple Support forums. Best explanation I could find (though not certain at all): the culprit of the number increasing particularly high is Bonjour. It’s a protocol Apple uses to discover devices and services on your LAN. It may be a bug or simply an oddity where Bonjour sees/discovers/somehow uses the same existing iPhone(71) for behind the scenes discovery of devices and services. So under certain home network setups/conditions, Bonjour changes it to iPhone(72) to keep a discovery clash from happening. Rinse and repeat again and you’ll see iPhone (74). Again this all theoretical.
Best fix was suggested by Annie_M, change the names of your devices to not be iPhone or iPad or (default Apple device name). That should stop it from getting these high (72) numbers.