[paul.fr]

assuming it is a standard sized, party balloon. how much weight can one lift, and how many would it take to lift a car from the ground?

[eric l]

A helium filled balloon would lift about 1.25 g per litre volume (for the balloon), but this would include the weight of the balloon. 
A filled balloon would measure about 5 litre, counting its own weight and that of the string you use for attaching the object in question, it would lift about 5 g.

[daveshorts]

so about 200 000 would be needed to lift a tonne of car

[syhprum]

I am afraid if you purchased what are normally sold as 'Helium filled toy balloons' you  would need a lot more as what you get is a mixture of cheap Nitrogen and expensive Helium just sufficient to make it buoyant.

[eric l]

And think about the amount of string you would need to tie the balloons to the car, and the weight of that.

[Bored chemist]

I had heard that so called helium balloons were filled with a mixture of hydrogen and helium. Hydrogen is much cheaper than helium but pure hydrogen is far too flammable.
Since helium is not a renewable resource, perhaps we ought to be pleased that people are reducing consumption this way.