Thank you for your question! and welcome to Gyan… 🙂
Visualize a circle. In that circle different souls come down at different “times” before completing the full circle. If you keep in mind that a human being could live up to 150 years as a maximum span, then you will realize that there are some souls who will take fewer births than 84.
Also, keep in mind that at every birth, we have different roles which call for different life spans. For instance in one of my 84 births, I may have only lived for 1 day. The next one 120 years, etc.
Therefore, according to gyan, the maximum number of births in a period of 5000 years which is a cycle is 84 births. Some souls may take less births to a minimum of 1 birth. Thus, every soul except God, takes at least 1 birth.