How high will a party balloon filled with helium go? How high can a party balloon filled with helium go before bursting?  If you filled 2 balloons to different amounts and tied them together, would they go higher?
I'm asking because I'm considering buying a 200g weather balloon to take a phone into the upper stratosphere, but I suddenly wondered if a bunch of cheaper party balloons and string would do the trick.
 A: There's a lot more than just physics to this answer.
First - you may not be aware of the fact that the "helium" you buy in a party store is a helium/air mixture that contains enough helium to lift a party balloon - but it doesn't have a lot of "lift" beyond that.
Second - you are in essence asking about the pressure at high altitudes; a balloon that has little elasticity (like a proper high altitude balloon) but a lot of "give" will expand in volume as the surrounding pressure drops, according to $\frac{PV}{T}=\rm{const}$. The pressure of the atmosphere follows a roughly exponential shape, with the value being about 0.1 atm at an altitude of 20 km (which is "somewhere in the stratosphere", depending on your definition).
Now there are two problems. The first is that your party balloons can only be partially filled at 1 atm if you don't want them to burst at 0.1 atm; this means you need a lot more balloons to get the same lift, and that means a lot of additional surface area / mass (including the strings holding the balloons together).
The second problem is the elasticity of the balloon. As the balloon fills, there is some internal pressure that builds up. This will compress the gas inside, making it denser and less buoyant. For a meteorological balloon, this effect is small
This is the reason proper high altitude balloons are slack - they can increase in size as they go higher. Having a single large surface rather than lots of smaller surfaces also significantly reduces the total mass of balloon you would need.
Finally - when toy balloons burst, they end up in the environment, and they kill wildlife. Please don't contribute further to that very real problem.
