Mythbusters is a great show and it is full of useful information.

For instance, will it take more ping pong balls under water, or 11″ helium
balloons in the air, to raise 1 pound of weight?

It takes 15 ping balls, submerged under water to float 1 pound. Those same
ping pong ball can withstand 60 psi, which is 4 atmospheres, or the
equivalent to 90 feet under water, just in case you were curious. They
expected it would take 50,000 ping pong balls to raise a small sail boat.
It ended up being 27,000 balls. The theory was the boat was buoyant too. I
wonder if was the salty sea water.

For the much larger 11″ helium balloons? Each balloon can float up 10 grams,
so you will need 46 balloons to lift 1 pound. If you wanted to lift a 44
pound girl you would need 3500 balloons, although they originally estimated
2070 balloons.

Just in case you ever need to raise anything.