The only thing that SQM offers is per internal IP fairness so that each active IP address get an equal share of the available capacity. That is with only one active machine it would get 50/20 with all 20 active each would only get 2.5/1, but it is rather rare for all machines to be active all of the time.
See the sing and dance section here if you want to try that. But again this is NOT exactly what you asked for and might not be acceptable for your requirements.
The total bandwith not get more than 50/20 mbps: This is working configured in the WAN interface.
All user have same priority, I don't want to have one massive user that could affect to other users, this is way I want to assign 5/2 mbps for IP. Maybe this is not necessary? By default the traffic is shared equal to all clients?
I am new with SQM, I see there is lot of tecnical reading about it.
That is exactly the kind of guarantee SQM delivers if configured as described. It will try to serve each IP-address equitably, sharing capacity between nodes that actually require data. In theory that can be gamed if a user on a machine creates a lot of IP addresses, but while possible that is not terribly straight forward to implement. It mostly does the right thing with minimal configuration required.
But please note that making SQM/cake do this will cost you some CPU cycles, but for a 50/20 link most router's made in the last decade should be powerful enough.
Please do not hesitate to ask questions and also, if you try it, report your experience (bad or good) back to the list.