How many cubic feet of hydrogen to lift 50lbs?

-- The density of air at 1 atm pressure and 0°C is 1.2922 kg/m3.

The density of hydrogen under the same conditions is 89 gm/m3.

The difference is 1.2032 kg/m3.

-- On or near the surface of the Earth, that 1.2032 kg weighs 2.6526 pounds.

-- For 50 pounds of buoyancy, you need 50/2.6526 = 18.85 m3 of buoyancy.

-- That's 665.7 cubic feet.

-- That's a weightless, spherical balloon 10.8 feet in diameter.

-- All of that just exactly lifts your 50-lbs off the ground in sea-level air.

If your balloon is not weightless, and if you want to keep rising as the

air gets thinner, you'll need some more.