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...