How many spermatozoa will be formed from 100 primary spermatocytes?

One primary spermatocyte undergoes meiosis I to produce two secondary spermatocytes.

Each secondary spermatocyte undergoes meiosis II to produce two spermatids.

Thus, a single primary spermatocyte produces four spermatids.

Each spermatid differentiates into one spermatozoon through spermiogenesis.

Therefore, one primary spermatocyte yields four spermatozoa.

For $100$ primary spermatocytes, the number of spermatozoa formed is $100 \times 4 = 400$.

Answer: $400$