On the scalp, there are approximately 100,000 hair follicles. Hair follicles go through well-established repeated cycles of development and growth (anagen), regression (catagen), and rest (telogen) to enable the replacement of hairs.
Over the course of three years, every follicle will produce a hair that grows, rests, falls out and then regrows. That means that every 1,000 days, most people shed 100,000 hairs. This equates to 100 hairs shed every day.
Hair is in a constant cycle of growth, rest and renewal so as long as the new hair that grows from that follicle is the same as the one it replaces, the hair density will remain constant. In patterned hair loss, hormones affect the follicle, making it smaller and the new hair becomes shorter and finer than the one it replaces. Eventually, the new hairs are so short and fine they become invisible and the scalp becomes bald.