Technically, you'd need a lot of balloons or a few really, really, really big ones. It takes about one quart of helium to lift just 1 gram or 0.03 ounces of weight. So in order to lift a human, you'd need a lot of helium -- about 13,000 gallons to lift a 110-pound person (50,000 liters to lift a 50-kg person). And you'd need a little more than that if you wanted to make the person rise and not just hang slightly above the ground. A regular amusement-park type balloon is about 1 foot (30 cm) in diameter, and it holds around 3.7 gallons (14 liters) of helium, so you'd need some 4,000 of these balloons to get your 110-pound person floating.
Source: Curiosity by Discovery
