About a 1000 will do but thats expensive.
For rough estimates figure the following:
3 to dry at most.
3 to remove AIO
3 to remove SG
4 to remove Blitz
But to make sure buy 9,987 more.
My motto is you cannot have enough. You will find more uses for them like the interior, wheels, windows, faucets, showers, wifes behind, etc etc etc.
Start with 10 or 20 and see how it goes. Get the detailing ones and maybe a few of the exceptional ones too.