For all practical purposes the quantity is infinite. The so-called "proven" reserves currently stand at 6 million tons [1]. But the quantity of uranium in seawater is staggering, of the order of 5 billion tons. It is estimated that, if needed, one could extract uranium from seawater at about 10 times the current cost of getting it from mines. That would add less than 1 cent to the price of 1 kWh of electricity generated by nuclear power plants. For comparison the average retail price of 1 kWh in the US is about 17 cents. Of course, nobody is seriously thinking of getting uranium from seawater because there are much cheaper ways to get it from mines. But any talk of uranium lasting only a few decades, or centuries is non-sense.

[1] https://world-nuclear.org/information-library/nuclear-fuel-c...

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.