how many kilograms a balloon could carry?

assume the balloon itself weighs nothing at all, and is filled with vacuum. The absolute maximum the balloon could support is the weight of air the same volume of the baloon, so if the balloon is, say, 1m^2 it could support the weight of 1m^3 of air (approximately 1.2kg at 20C at sea level)

of course in a real situation you have the weight of the balloon to consider, and also the weight of whatever gas is filling it, so a reasonable approximation is probably about 1kg for 1m^3 balloon. 1 litre of water weighs 1kg, so there is a nice approximation!

I weigh about 90kg, so to hold my weight, a balloon would need to be almost 100m^3!

Do you mean a helium balloon lifting some water?

Density of Helium gas 0.18 kg/m³

Density of Air 1.28 kg/m³ at 0ºC and 101kPa

difference between these two is the weight a balloon can lift. In this case about 1.1 kg per m³ of volume.

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If you consult the web site below, you will be able to calculate how many balloons you will need to lift a given weight.