Jamhuery that is impressive with the exact numbers. I was just going to walk through how to solve without numbers because I surely wouldn't have known that without extensive research... Anyway here is my take on it:
First you would need to figure out how heavy the house is. This isn't an easy task to find a scale large enough to weigh an entire house. Rather I would take the individual components and sum the weight of the materials used to construct it.

Next, The Balloons. How much weight are they capable of lifting and at what speed? The simple F = ma can come into play ( Force equals Mass Times acceleration). From this photo alone, we do not know how quickly the house is rising. More balloons = a faster lift into the sky because there is more force opposing gravity's pull on the mass of the house ( IE Weight). Similarly to what Jamhuery mentioned you figure out how much helium is contained in one balloon. Then figure out the air mass compared to oxygen. The tricky part to this is that it will change the higher you go because of the density of atmospheric pressure and additionally temperature. Heat causes air molecules to expand, while cooling causes them to condense. Think of a car tire. When it is hot outside, your air pressure in your tires is high, when it first starts to cool every fall/winter, your tires appear slightly less full and the pressure is decreased.

Once you have figured out how much 1 balloon can lift under the circumstances given and how much the house weighs, and at what rate you would like the house to lift, you can determine how many balloons you need.

To Actually make this happen, it is going to take a whole (excuse the french) Shit ton of steemians to get this done. Huge numbers of balloons will need to be filled with helium within a very short period of time (as we are all familiar with it only takes a couple days for helium balloons to fall to the ground and prior to that they are constantly loosing force, which would then necessitate the use of more balloons to make up for the loss due to inflation time. Using Jamhuery's number of 69 balloons for 1 kg and assuming the average house weighs roughly 50,000 Kg ( This I googled for a very rough estimate, as all houses vary) That is literally over 3 million balloons. But that number only holds if these are inflated simultaneously...

How long does it take to fill a balloon with helium and connect it to the house? Well once we figure that out, then we can figure out exactly how many Steemians we would need. Lets say we are incredibly efficient and can complete a balloon and attach it in 40 Seconds. We probably want to have this project done in less than 5 hours so the balloons don't start sagging (as those in the front of the picture are doing). One steemian can do 750 balloons in that timeframe.. With needing 3 million We would need 4,000 steemians who are all equally efficient at inflating and attaching balloons.

So In sum (and after making several assumptions)
We need lots of mathematical numbers to plug and chug through equations, but more importantly we need the Man power to do this efficiently otherwise the number of balloons changes with variation in effort and time. In fact countless variables throughout this can change the number needed to make that happen!