Man, guessing it all depends on what size vehicles your dealing with, the type of wash you offer and also your process.
I have a 150g tank but only fill it to a 100g. My process is thorough rinse, foam gun soak, two bucket wash and final rinse. I also have a third 5g bucket for wheels and tires.
Depending on the condition of the vehicles I can usually squeeze out 10-12 Complete washes.