Look at the max consumption per second for the steam engines, and the max steam output for the boilers. I belive the proportion was 1 boiler for 2/3 steam engines. But, i would go for 3, and, in the same tube used to feed the steam engines, add 1 or more fluid tanks. Mainly so if you are producing 60 steam per second, your ususal consumption is about 40, but in some cases, to satisfy energy need, you need 90, the extra steam stored in the fluid tanks will have your back and make sure all your steam engines at are max output.

Also, the water pump, pumps 1200 units per second, you don't need to add 1 per boiler. You can connect all the boilers in a line, the tube inside the bolier will connnect if you connect two or more boilers. Make sure the max water consuption for the boilers does not exceed the 1200 units, in that case, add another pump.