Short answer: up to 5 computers and up to 10 for the total of computers and mobile devices combined.
To use Match, you have to "associate" a device or computer with your AppleID. You can associate at total of 10 computers and iOS devices *combined*. You can only associate *one Apple ID* with each device or computer, and this can only change once every 90 days.
To play the music on a computer, you must additionally "authorize" the computer. You can authorize a total of 5 computers to play the content. You can authorize music for multiple users on one computer, and change the authorized computers for an AppleID. You can deauthorize computers and authorize different computers as often as you want *if you have access to them*. Otherwise, you would have to "deauthorize all", which you can only do once a year.
I hope this helps.
If you read that carefully, you see that you *could* actually run Match on up to 10 computers, but once you go over 5, you would have to authorize the computer each time you wanted to listen to music and then deauthorize it afterwards.
Here are the detailed policies on authorizations: