How many kegs/ liquor to buy?

Having a party of about 25-35 people. I'd say about 20 people could be considered heavy drinkers, as there will most likely be drink games and keg stand going on. Most people will be drink beer, but I'm buying a liquor for the people who dont. How many kegs should i get, and how much liquor to buy? Safe to say the party won't start till 8 or 9. (told people 7).