I don't know the exact numbers for each of the certifications (our database has gotten so big that it often times out when I try to run such queries - definitely something we need to optimize once we get time, but we have a lot of competing priorities).
I can tell you that 10,000s of people have earned certifications, and at this point probably hundreds of people have earned the Full Stack Developer Certification that you get when you complete all 6 certifications.
Also, more than 40,000 people have gotten their first developer job after completing at least some of freeCodeCamp's curriculum.
As for the interview prep section – we started the #ProjectEuler100 certification to encourage people to get more algorithm and data structure practice. So far people have gotten far – maybe 60 problems into it. But nobody is anywhere near completing all 600+ Project Euler problems and the hundreds of Rosetta Code problems. I don't know if it's humanly possible. Maybe you or someone reading this can prove me wrong :)