Depends on what you mean. If you mean "how much is commercially viable" versus "how many kilograms of the stuff are there" you get very, very different answers.

For just "how much stuff is there", we can look at the concentration in the earths crust, and we get about 10^17 kilograms of the stuff (10^15 if you want U-235).

If you just say "Sure, but we can't get most of that, how much can reasonably get extracted at current prices and technology" then you go down to about 10^9 kilograms.