Does an apple music subscription cover the Mac and iPhone? or do you need two Subscriptions?

One will work with both as long as both devices are using your Apple ID. The subscription is per Apple ID, not the hardware in your case of those two devices.

As long as you have Sync Library enabled on all devices. It will all sync with one subscription (same Apple ID)

Turn on SyncLibrary with Apple Music

https://support.apple.com/en-ca/HT204926

If you subscribe to Apple Music, you can turn on Sync Library to access your music library and music that you've downloaded from Apple Music on all of your devices.