how many ring or radial socket makes a circuit
The number of sockets you can have connected to a circuit isn't limited by the circuit breaker's amperage. No matter how many sockets you connect, you're still limited by the 20A limit on the circuit breaker. A socket with nothing connected to it does not draw any current.

Think about what devices you want to connect to the circuit. Find out how many amps each one draws and total that up. A 20A circuit can supply 20A intermittently or about 16A continuously (load for more than 3 hours at a time). Use the total amperage you need to supply to determine the number of circuits you need.

If you wish to connect multiple devices, but not use them simultaneously, you may do so as long as they don't exceed 20A or they'll trip the breaker.