Although it's impossible to give a for sure size for all needs, as grain bills and boil times for each beer can vary widely, below is a good rule of thumb for each Spike System:

 

10gal Spike System -  2.5-6 gallon batch

15gal Spike System- 5-11 gallon batch

20gal Spike System - 6-15 gallon batch

30gal Spike System - 10-20  gallon batch

50gal Spike System - 15-32 gallon batch