If you were reading it out to somebody, eg. to write down in a lecture, I would say N-sub-zero.
But if this is a particular mathematical term then it's probably N-nought, or N-null

eg. the set of cardinal numbers ![aleph-null][1] is normally Aleph-nought or Aleph-null


  [1]: https://i.sstatic.net/c4UPF.png