indexing valve, all zones on at once
It appears that your indexing valve relies on downstream pressure for the valve to seat properly. Too many sprinklers reduces the pressure between the indexing valve and the sprinkler heads. 

You ask: 


  How can one determine the proper number of heads for a given zone?


The true answer is that the system should have been designed to provide  adequate water volume delivered at the listed minimum operating pressure for those sprinklers. This is usually a calculation using static and flowing pressure readings with consideration of pipe diameter(s), lengths, and system configuration, accounting for friction loss.

In your case, I would suggest a simpler method: remove one sprinkler and cap the riser, then see if the indexing valve works. If not, continue to remove and cap sprinklers until it does. When it does, make sure the sprinklers are spraying correctly; just because there is enough back-pressure to allow the valve to operate does not mean your system is necessarily at the pressure required for optimum sprinkler head performance.

I would also suggest you look into a higher capacity pump, if you absolutely need to run more heads.