What size water pump do i need?

I have built my own spray rig to water some stock and tanks and a garden. It is a 330 gallon tank and has a spout at the bottom that i am going to connect a multi purpose pump to. My question is what size pump would i need? I have been looking at Atwoods and Tractor Supply and have seen some 2.1 GPM and a 5 GPM pumps i just don't know which would best suit me. i am planning on having the tank on the trailor and running water hose to a sprinkler or two. I would like to know how man sprinklers are possible on this type of system and still keep good pressure. Thanks for any answers.