Photon ccu question

I want to add a multiplayer mode for a game I am making( the local multiplayer aspect is already complete).

What I could not understand after researching through multiple sources is what is ccu and how it works.

I would love if anyone can help me understand if I give the details on the kind of multiplayer game I am working on, it would be really appreciated :).

It is a 1 vs 1 game and the multiplayer aspect I am thinking of is pretty much like the networking in Miniclip's "8 Ball Pool".

Where there are random or friend request matchmaking.

is photon reliable for this type of multiplayer game? how many players (total) can play in my game with the 20 ccu?

I am also thinking about the Unity networking but am afraid of the whole free for a limited time aspect, should I use unity's networking instead?