Do you know how much disposable income there is in the iPhone-having segment of the US? I’d say that’s a very reasonable target to start with.

Also, other rich countries, like say some north/west European ones, have a large (~50% or more) share of iPhone users as well.

If you need to go to market quickly, it makes sense to start with the trendy people with money, then you can go for the long tail of Android users.

Edit: Clubhouse isn’t an app where it matters whether every single one of your friends and family is on it for it to be useful/entertaining to you. And if you become established in the most desirable segments of some major economies first, you have a good foundation for going after the rest of the world later. There’s no need to try to cater to every single potential user on Earth from the get-go.