How many Safety Valve on each boiler?

I would say One is enough, but it depends on what kind of boiler. But only one at the tank to control the pressure due to the temperature

Correct, the super heater outlet will have one to three safety valves on either side of the boiler to prevent explosion due to excessive pressure of steam.

There are normally two to six safety valves provided in the drum depending upon the capacity.

we connect2 saftey valve and set values are different

 To provide a designed-to-fail pressure-relief valve so that the entire system does not explode. In general, redundant systems are designed such that the first one fails at a system-wide safe level, the second one fails at some specified point below the failure point of the boiler itself. For example, the heating system in our house has elements within it that are designed for up to3 bar of pressure. The boiler itself is designed at4 bar. The TPRV is designed at2 bar @120C as a single device to cover the boiler/heat loop. However, the internal domestic water heating loop TPRV is set at7, bar but only95C and the loop is designed at10 bar.         

I think between2 to6, but mostly2 safety valve.... thank you Mr. Alex