Pardon me if this is silly, but as per my limited observation, the number of APIs is higher than the number of clients that exist. How are APIs different from clients?
Ethereum doesn't have an API, as it is understood in the traditional web. It has only one yellow paper which outlines the methodologies which should be implemented by all clients. However, we live in an imperfect world, hence sometimes different clients find or produce different problems (see geth and parity).
Despite the aforementioned clarification, I think you are referring to the asymmetry between light nodes and full nodes. It's hard to track these numbers, we run a distributed network here after all, but ethstats.net provide some pretty good rough estimates.
The number of light nodes is definitely higher than full nodes, but the former are extremely useful to the ecosystem. They prevent full nodes from colluding, because the client software should be forked too if one wants to have users accept a malicious split. Vitalik has written a great article on coordination problems.