The physical dimensions of the Tsar Bomba are on the wikipedia page:
https://en.wikipedia.org/wiki/Tsar_Bomba
If 2 million are needed, then that's 100 cubed, twice. So two cubes of this bomb, with each cube measuring 100 units per edge, filled solid. The bomb is already designed, so producing 2 million of them would be a matter of setting up an assembly line, and churnning them out like automobiles, once the raw materials are procured.
Each one is about the size of a car, so a parking garage that holds 2 million cars is an okay frame of reference. Most stadiums have parking lots that hold 10,000 spaces, so 100 stadiums, across one hundred cities, but all together in one place. Realistically, though, two million cars easily fit inside any large city.
As for the gaseous concentration of fusion gas in the ocean, that's a much tougher problem to tackle. Really, because the huge fusion bombs that use deuterium (and tritium) are sourcing that material from the oceans (or other bodies of water). Culling deuterium from natural water sources is among the easier ways to produce weaponizable amounts of the gas. So, in order to create twenty times the natural standing concentration would require synthesizing your own new, man-made deuterium from the normal variety of hydrogen, by irradiating non-heavy water.
For every million gallons of ocean water, you can pull out maybe 156 gallons of heavy water, so you'd get about six hundred trillion gallons of heavy water, if you could capture all of the natural heavy water in the world. Take 600 trillion gallons, and multiply it by twenty. Bingo, you're there. That's the ignitable concentration, if you had 2 million Tsar Bombas.