Let's make it clear right away that we won't need balloons with just air, but with helium. Helium balloon vendors claim to be asked this question quite often. In fact, to calculate the number of balloons that can lift a person is not difficult. To do this, you need to know the lifting power of helium and the exact weight of the person or object to be lifted.

It is known that 1 cubic meter of helium is approximately 1 kg. Then 1 ball with a diameter of about 27 and a volume of 8 liters can only lift 4-5 grams, because part of the lifting power is spent on lifting the weight of the ball and thread itself. Thus, to lift a candy bar weighing 100 grams you will need 100/5 = 20 balls.
To lift a man weighing 70 kg into the air you would need about 14,000 balls! Imagine a balloon bundle the size of a two-story house, about 7 meters in diameter. Although, to lift a child in the air 5,000 balloons in a bundle about 5 meters in diameter will be enough.
Do not forget about such factors as weather conditions: snow, rain, wind, too high or too low temperatures. All these are not the best conditions for helium balloon flight, because they greatly affect the lifting power of the balloon. For example, ordinary helium balloons, even in light rain, simply will not fly because of the water droplets stuck to them.