>>20
Sorry, only one level of nesting is possible at the moment. The original Emacs was a set of macros for TECO, and I remember seeing a TECO implementation in Emacs Lisp, but as there's no TECO implementation in TECO Emacs, that's as deep as you can go until someone re-implements a Lisp-capable Emacs in Emacs Lisp.
You can run Emacs in Emacs term-mode, but that starts a new process for each nested Emacs, so it doesn't count.