Your pump needs to be designed or specified based on your need or gallons per minute. You also need to take into consideration your elevation difference from the pump to the sprinkler heads. How many feet, What is the pressure requirement of the sprinkler head. What is the gallons per minute you want to keep flowing through the pump. Break it up into 2 or 3 zones, you don't have to run all 15 heads on 1 zone. Choose a head good with low pressure like find out how many gallons per minute it uses on 45 to 50 pounds of pressure. Add up the gallons per minute until you get to 10 or 12. Break to a new zone or valve and do it again. Repeat and that will give you your number of zones.
Good luck.
This is basic I don't know your particulars, but this should get you started.
So are you saying I NEED to break it into zones? Because I'd rather not... for simplicity, i'd love to have all the heads run at once. The impact head I was looking at has these specs:
Performance Chart
PSI -- Radius -- GPM -- Bars -- Meters -- M3/h
25 --- 35 ------ 2.8 ---- 2,0 -- 11,0 -- 0,68
35 --- 38 ------ 3.3 ---- 2,5 -- 11,7 -- 0,78
45 --- 40 ------ 3.7 ---- 3,0 -- 12,1 -- 0,83
55 --- 41 ------ 4.1 ---- 3,5 -- 12,5 -- 0,89
That head uses around 4 GPM at the PSI you suggested... So, is it as simple as taking 4 GPM x 15 heads and getting 60 GPM pump requirement? (btw, my pump, water and heads are all about the same level.