There's more Thorium in the crust, there's more Uranium in seawater. With either fuel (plus breeder reactors), the nuclear fuel supply of earth is unlimited on a several billion year timeframe at world-scale.
Here's an entire special issue (from 2016) of Industrial & Engineering Chemistry Research describing progress in extracting uranium from seawater: http://pubs.acs.org/toc/iecred/55/15
Neither does Uranium if you're using breeder reactors. And you have to be using breeder reactors if you're using Thorium because Th-232 is a strong neutron absorber and you can't start a reactor on it at all. You have to breed it up through Pa-233 to U-233 which is fissile. By definition all thorium reactors are breeder reactors. Fertile U-238 can do the exact same thing; instead of U-233 it breeds fissile Pu-239. If you want all the raw details on Thorium check this out [1].
