Question
How many rainbird popup heads can i put on one zone?  My front lawn I am using rotary popups and they are doing fine. My back lawn on one zone I am having problems with the presssure and the heads fully popping up. I started with 16 pop up heads cut down to 10 now to 8 and still seems to be too many.


my last sprinkler system on one zone handled 20 pop up heads at one time.

Answer
Thanks for the question. This is a very common question with a complex answer.
The number of heads depends on your water supply, pipe size and nozzle selection. How many rotary heads are you running in your front lawn? If you are operating 5 using 2.5 gallons each then you have 12.5 gallon per minute.
A common misconception is the small pop-ups use less water. Some of the nozzles actually use more than the rotary sprinklers. An example is; a Rainbird 12' full uses 2.4 GPM a 15' full uses 4.0 GPM. If you were using 5, 12' full pop-ups it require 12 GPM. Now change the nozzles to 15' and the water required would be 20 GPM. It's a big difference for only 3 additional feet of coverage.
If you can tell me the number of rotary sprinklers and nozzle size you are using per zone I can recommend an approximate number of pop-ups you could run.  
I apologize for not answering sooner, I'm on vacation and the Internet service here isn't too good.