One, or a small number, and because this is the history of these types of things.
> The term "federation" is borrowed from political science for a reason. Just like in the USA, multiple federated states who share broad-strokes policies maintain interoperability while also maintaining their own sovreignty.
The USA is actually not a federation, and purposefully stopped being one (it turned in a confederation, removing the right to leave, effectively centralizing it under federal control) when the country was founded.
(Edit: I reversed the terms confederation and federation here, but what I mean is that the founding of the US removed sovereignty from the states.)
This is where the rubber meets the road: any decentralized system will need to be able to answer the question "how do we [the govt, usually] forcibly remove content from the network?" The way you answer that question will be the way the network centralizes.
The history of true confederations (Colonial States, Personal Computing, the Internet) is that they are temporary chaotic moments before we revert back to the mean of centralization (United States, SaaS/FB/Gmail/centralized communication and iphones as tracking devices, PRISM).
This is all an instance of https://en.wikipedia.org/wiki/Conway%27s_law. Our culture is not confederated, and so we wont be able to build scalable confederated systems within it.