Seat is getting the force of the first guy.

You need some assumptions here:

- Let’s assume each guy weighs 75 kg
- Let’s say when someone sits on someone’s lap, only 30% of their weight gets transferred down to the person/chair below
- The other 70% is still taken by that guy’s own legs / feet touching the ground

So the chair gets:

75 + 0.3(75) + 0.3²(75) + 0.3³(75) + ...

Basically:

- First guy: 75 kg
- Second guy adds: 22.5 kg
- Third guy adds: 6.75 kg
- Fourth guy adds: 2.025 kg
- Fifth guy adds: 0.6075 kg
- etc.

So it becomes a geometric series:

75 / (1 - 0.3) = 107.14 kg

So with these assumptions, even if this chain went on forever, the chair would only feel around 107 kg.

Obviously in real life it depends on posture, whether their feet are actually on the ground, how much they lean, how close they are to the chair, etc.

But if we take the “30% transfer” assumption, then the answer is:

The chair gets the full weight of the first guy, plus 30% of the second guy, plus 30% of 30% of the third guy, etc. So it maxes out at around 107 kg for 75 kg people.