It depends on what size the balloon is.
The standard size of a balloon is 11 inches and has an approximate volume of .403 cubic feet.
Nitrous Oxide or N2O, has a volume of 8.73 cubic feet per pound. A 20 pound gas tank can store 174.6 cubic feet of N2O.
Based on those information, we divide the volume of a 20lb. tank (174.6 cu. ft.) by the approximate volume of the balloon (.403 cu. ft.). The number of balloons you can fill is around 433.
Nitrous oxide, sometimes called "laughing gas" is a clear, colorless, oxidizing liquefied gas with a slightly sweet odor. Nitrous oxide is a safe and effective sedative agent that is mixed with oxygen and inhaled through a small mask that fits over your nose to help you relax.
Nitrogen gas can cause dizziness and affect an individual’s judgement. It can also – in more extreme cases – cause unconsciousness and death from lack of oxygen. Learn more about its danger at Independent.co.uk.