Balaji introduced the Nakamoto Coefficient several years ago:
The basic idea is to (a) enumerate the essential subsystems of a decentralized system, (b) determine how many entities one would need to be compromised to control each subsystem, and (c) then use the minimum of these as a measure of the effective decentralization of the system. The higher the value of this minimum Nakamoto coefficient, the more decentralized the system is.
He also tried to perform an initial analysis of the coefficient for:
What's the Nakamoto Coefficient for Cardano? Charles regularly claims Cardano is (among) the most decentralized cryptocurrencies in the world - has anyone tried to quantify that?