I probably use about 5-7.
It depends on park hours. But if you get one right away the window will open within the hour, the second one in may an hour, so you get one at T=0, one at T=45min, T=1.5 hours, T= 3.5, T=5.5, T=7.5, T=9.5, then on a long day there is T=11.5, T=11.5, T=13.5, T=15.5.
But this is assuming that return time build past the two hour limit, and that you are picking yours up right at the 2 hour limit.