This is determined by knowing the amount of heat required by each zone then adding them up. The total heat required in btuh, divided by 10,000 is the required gpm the pump must deliver at the highest resistance the pump must encounter. This gpm capability is found in the pumping charts published by pump manufacturers.
There are helpful design computer programs online to give you the maximum gpm flow of pumps through 3/4 and 1/2-inch tubing.
Harold Sizing this system ONLY by the BTU requirements and NOT taking into consideration
1- Temperature of the water
2- Velocity (FPS @ 220)
Too much of either will cause all kinds of great stuff like fast erosion and noise and guaranteed system failure.
Being more conservative I like to under size the actual demand in GPM so if an apprentice or stumble bum ditch digger or Home center escapee should decide to raise the water temperature with an indoor out door set up the higher temperature wont have adverse effects on the piping