The API is supposed to be available once 13.5 goes live (and it is available in public beta now, I believe). Here is a list of states who are participating with compatible apps: https://9to5mac.com/2020/05/21/covid-19-exposure-notificatio...
TL;DR: Hardly any.
Edit: Whoops, I am out of date, I thought 13.5 was not out yet! Apparently it is, and then some.