This would be a stack size limitation. Larger stack size == more nest levels.
Note, the above is for runtime. At compile time, of a single procedure, the compiler may have an internal limit, possibly limited by its stack size and/or internal structure design for tracking tokens (variables).
If you are running into a problem, please state the particulars.
Considering the high-level of nesting in your loops, have you looked into recursive algorithm(s) and evaluated whether they can help you with your computing needs?
Starting Fortran 2018, unless marked as NON_RECURSIVE or dictated by some compiler option(s) to override the standard, every procedure is recursive. And one can thus hope a Fortran 2018-compliant compiler will be reasonably good (and striving toward being highly adept) at optimizing the program with respect to recursion, both in terms of run-time (e.g., stack usage) as well as compile-time processing.
Thus investment in recursive algorithms toward one's computing needs can prove valuable in the long-term.