Helium balloons will float because of the fact that helium is lighter than air. When filling a helium balloon you should know how much helium you need for the quantity of balloons you are ordering.
The most common size of helium balloon is the 12 inch. The 12 inch balloon will inflate to the size of a basketball. This 12 inch balloon will require about 1/2 cubic foot of helium per balloon. So if you have 250 12 inch balloons you will need about 125 cubic feet of helium. The 10 inch balloon is slightly smaller and will require about 1/3 a cubic foot of helium. So just multiply 1/3 or .3 by the number of balloons and that will give you the amount of helium you need for your helium balloons. So if you have 500 10 inch balloons, multiply 500 times .3 cubic feet and you get 150 cubic feet of helium needed.