The calculation of your life times left is based completely on your desires—specifically, your karmic desires.
Karmic desires—to understand human lives—depend solely upon your karmic desires.
Karmic desires are nothing but a collection of desires due to which a being (the being itself is a collection of memories) takes birth in this world. How can one know these desires? Simply put, these are the desires that will not leave you alone no matter what. For example, if you have a desire to build a strong body, this desire will persist with you all the time, even on your deathbed, and we all know that such a desire cannot be fulfilled on a deathbed. We also know that this desire has its timeframe—which is in our younger days when we can achieve such a goal—but if that being is not able to achieve the goal, it will persist with that being even after death. So, there is a high chance for that being to extend the date of its freedom from human life and keep taking birth until this desire is completely fulfilled. You see, finding your karmic desires is actually important because there are desires that are not the karmic desires of the being; they come from society, or a competitive and uneducated society that gives much more importance to competition instead of fulfilling your desires.
This is the reason why a student of spirituality generally stays away from society, because this society itself is wholly based on competition and nothing more. Parents are generally the reason for deciding everything for their children; these types of parents do not know it, but they are destroying the destination of their own children.
How do you know your karmic desires? These desires will persist deep inside your heart, and you probably already know the answer. Because these are your desires, nobody can find them for you. You have to decide on your own. Based on those desires, you can easily guess the number of lives you have left in the human world.
Example: A being has two karmic desires left—one is to become tall and masculine, and the other is to look cute and short. Both of these desires are not possible to cover in one lifetime. Therefore, that being has to take two lifetimes. However, if these desires are something like "I want to become tall and cute," then it is possible in one lifetime, so the chances of that being fulfilling its karmic desires in one lifetime increase greatly. The only thing that can hinder it is probably this society and that being's biological parents. But if your karmic desires have high intensity, then nobody can stop that being. How that being increases the intensity of its karmic desires depends heavily on the environment and good parenting, because if parents understand the desires of the child, it becomes easy.
This is how you can calculate your lifetimes in this human world; it is pretty simple. The best thing is to first know your karmic desires, not the desires that come from the outside world. Becoming rich and showing off is generally the main desire that comes from the outside world or a competitive mindset. According to me, a competitive mindset is not great for your own good—or, I should say, for the being—because you are not just a being; you are existence itself.