Only 200 out of 2 000, 000 sperm cells reach the ovum. And only one (in exception to very rare cases of two) sperm cells actually participate in the process of fertilization. In comparison, an average female only produces 400 of egg cells during their lifetime, but the eggs cells are significantly larger than sperm cells.
The answer is logistic growth.
It is modeled by an S-shaped curve. Carrying capacity (K) is the maximum individuals of a population that a habitat can sustain indefinitely
given the limited resources in the environment. Before this number
is achieved, the population grows exponentially and slows as it gets close. When
the K number is reached, the stationary phase enters.