One average-sized balloon; thirty medium sized balloons; or 100 of the regular-sized ones.

Into space?

Forget it. A balloon can only lift anything through air by its buoyancy, meaning that it can displace as much air as weighs at least as much as the balloon plus its payload. In space there is no air to displace. Therefore the balloon can't reach that high. 

Specially designed balloons can in fact reach high into the stratosphere, in fact I seem to remember unmanned balloons going over 50 km up. That is pretty good going. But it is not space.

Louise, what Jon and I are trying to say is that your question is not very clear.

First, an average-sized balloon would be quite different for a meteorologist and a fairground salesman.

Space can have different meanings too. Do you mean "off the ground" or do you mean "outer space"?

If your asking how many balloons will get you off the ground I can quote data from 2 television programs.

• Mythbusters lifted a 20kg child with 3500 party balloons.
• Brainiac lifted a 92kg man with some large weather balloons that they claim is equivalent to 30,070 small balloons.

It is impossible for any number of helium balloons to lift anything into space. The lift capacity of a helium balloon is the mass of the air displaced but the voluum of the balloon minus the mass of the helium minus the mass of the balloon and the mass of any payload. As a helium bollons rise the air pressure drops and the mass of the displaced air decreases. For any number of helium balloons there reaches a point there the air pressure drops such that it can no longer support the payload or the ballon bursts.