It's impossible to tell. No one can calculate what you'll hit, or how many you'll hit at a time. Therefore you can't find out how many chinchompas you'll need. Also due to that, you don't know how much time it will take, so you don't know how many prayer potions.
Your best bet is to just buy like 10,000 Chinchompas, 100 Prayer pots, and buy more when you run out.