If you're trying to develop a method of estimating how many hairs are on your head, it would be best to take on this task in two parts: background research and sampling.
Firstly, I suppose you have a reason for asking how many hairs are on your head that is based in something you heard, something you want to know, or something you want to prove wrong. For example, maybe you heard that women have more hair per square centimeter on their heads than men. Or, maybe you heard that we have 60,000 hairs on our heads (on average).
Step 1: Before you dive into this question, ask yourself what you want to know and why. There may be research out there that either talks about how many hairs are on your head or provides a way for you to get at the question thorough a specific method. After you conduct a literature review, you'll have a better idea as to how you will proceed with the next step.
Step 2: Take a sample and count hairs within that sample. Depending on your literature review, you may have many ways of doing this. For example, you may choose to take a 1cmx1cm square on someone's head and count all the hairs within that square, then generalize that to the rest of their head. But, how will you know you're accurately counting the hairs? How will you know your method works? There may be existing literature that walks you through best practices. You may also learn that different places on peoples' heads have different concentrations of hair follicles, thus, different amounts of hair. You may also determine that other factors such as age, ethnicity, health, gender, or location play a huge role in the number of hairs a person has on their head.
The combination of literature review and sampling should help you develop a method of counting hairs on your head (or someone else's) to answer your research question. Please take a look at the attached article that talks about counting hairs to answer a research question regarding hair loss (linked).