Ethereum is a decentralized platform that runs smart contracts: applications that run exactly as programmed without any possibility of fraud or third party interference.
Ethereum is still in development and subject to significant changes. The number of developers working on Ethereum can be difficult to estimate, but various reports and surveys suggest it is in the hundreds, if not thousands.
The majority of developers appear to be working on Ethereum-based projects rather than the Ethereum protocol itself. This is likely due to the fact that Ethereum is still in development and subject to change, making it difficult to work on the protocol itself.
However, there are a number of core developers who are working on Ethereum, as well as many more who are working on projects built on top of Ethereum.
In conclusion, it is difficult to estimate exactly how many developers are working on Ethereum, but it is likely in the hundreds or even thousands. The majority of these developers are likely working on projects built on top of Ethereum, rather than the Ethereum protocol itself.