Question
How many rotors can go on one zone. I have 40 psi 3/4 inch feed pipe.

Answer
Rob,
Sorry for such a long delay of answering, It's been busy in the irrigation biz lately.
With the pressure you have you can only get 8-9 GPM. This equates to 4- 2.0GPM rotors. The 2.0 is about as small as you want to use. If you try for more heads by using smaller nozzles you'll end-up with very poor coverage.

Matt