How high (altitude)a normal size party balloon filled with helium will rise in a

The density of helium is only one-seventh of the density of air at the same pressure, so when the balloon is stable, the other six-sevenths has to be accounted for by a combination of (i) the weight of the balloon fabric, and (ii) the tension in the balloon fabric keeping the helium at a higher pressure than the surrounding air. There is no simple answer, but four miles would not surprise me.