The CO2 tank is a critical part to any kegerator. It supplies carbon dioxide to your kegs.
The carbon dioxide keeps your beer carbonated and supplies pressure to dispense the beer.
The number of kegs will vary depending on your settings but this will give you a good estimate.
You have a couple of options.
One place to buy a tank is a local welding supply store. These tanks are typically made of steel and are part of an exchange program. You just drop off the empty tank and pick up another tank that is full.
You can also purchase a tank that is not exchanged but instead filled and returned. You will be required to maintain certification on these tanks.
There are several online stores that sell tanks. The tanks are typically made of aluminum and are sold empty. You will just have to take it to the welding supply store to have it filled.
My local welding supply store charges $13 to refill a 5 lb tank.
It's a good idea to have two tanks...You don't want to run out of CO2 during a party.
I have two 5 lb CO2 tanks for my kegerator. One is part of an exchange program and one is an aluminum tank that I own. The aluminum tank is definitely lighter in weight and looks better but the steel tank exchange program is more convenient.
You can't go wrong with either option, so don't worry too much about your decision.