I'm not aware of a limit in number of nests - the limit will probably be
either "Query Too Complex" or the expression being too large, and will vary
with the details of your expression.
If the question arises, though, then nested IIF's are probably not the best
solution for the problem; a Query joining to a lookup table, or a call to the
Switch() or Choose() functions, or a custom VBA routine may be better.