The 2000SC is the minimal collateral budget the UI asks you. If you want to offer a lot of space, you'll need more collateral, of course. When you host, you offer a collateral for every contract a renter forms with you. You choose how much, but if it is 2 or 3 times bigger than your price per Tb, you have more chances to be selected by the renter.

Once the contract is formed, that collateral stays locked. If during the time of the contract you keep a 97% uptime (trough periodical Proofs-of-Storage), you recover the collateral and receive the payment for hosting and bandwidth. If you fail, renter recovers his payment and the collateral is burnt.

Of course you can stop being a host at any moment, but the locked collateral will not be returned to you until the end of the contracts you previously had (and only if you keep the 97% uptime)