I agree some species have different numbers of instars depending on if they are male or female. Also sometimes mantises will skip a molt going right to adult from an earlier instar, I had that happen once with a Carolina male. He had 2 molts to go until adult but he skipped the last one, and went right to adult with wings that were shorter then normal. He was also small.
Size can also help approximate what instar it is, though it might vary depending on species and such. Such that in my mantises, a Chinese mantis that is 25-30mm about is a L4 instar.