Say the balloons are about a foot in diameter & you weight 160.
ok, so lets calculate this. a balloon with a foot diameter has a volume of .7854 cubic feet. .5 cubic feet converts into 14.158423356 liters. The density of helium is .1785 g/L (grams per liter) and the density of air is usually approximately 1.25 g/L. The difference between these densities is 1.07g/L. The formula for density is d=m/V (density equals mass over volume) so we set d=m/V, 1.07=m/14.158423356. the 1.07 was the difference in density(how much less dense helium is than air) and the V i said(14.158423356) is how many Liters i said was in .5 cubic feet before. it has to be in liters since the density measurement i gave you was in grams per liters, so for the formula to work we need to work in liters and receive an answer in grams. ok, so solving for m in that density formula, we get 15.95g. so each balloon can hold about 16g. but i looked up the average weight of a regular party balloon and it's 2g, so each balloon can hold 14 more grams. converting grams to pounds, 14 grams equals .030864717 pounds. so each balloon can hold .030864717 pounds. now if we divide your weight by the amount of weight each balloon can hold, we will get the number of balloons you need to support your weight. 160/.030864717 = 5183.9 so 5,184 balloons.
you would need 5,184 balloons to equal your weight. this only makes you weightless, so the force of these balloons would basically perfectly counteract the force of gravity, so you would need to jump to push off and you wouldn't be pulled back down. if you wanted the balloons to actually lift you, you would need more, and depending on how many more, you would have a greater acceleration upward. this is also assuming that each balloon is perfectly one foot in diameter, so the number of balloons may fluctuate a bit.
even so, might i suggest you use a balloon with a larger diameter?
if you want to find out for sure with specific balloons, you can always get like 50 balloons and some weights. then you can measure how much weight the 50 balloons hold. then you set up a proportion saying that 50 balloons divided by the weight you measure it can hold(in pounds) equals X balloons divided by 160 lbs. then you can solve more precisely for the specific balloons you have.