I have a lot of hard bumps on my scalp, especially at the back. i haven't noticed them since they do not hurt nor itch. they're about a half mm in size and at the base of the follicles. could it be cancer?

What you described suggests folliculitis, inflammation of the hair follicles. It is not suggestive of cancer. You can reduce your risk of cancer by adopting healthy habits: Eat a mostly vegetarian diet, avoid alcohol, tobacco and street drugs, avoid excessive exposure to sun, get HPV vaccine, exercise 30 minutes each day, drink enough water so your urine is mostly colorless.

Hard follicular bumps on the posterior scalp usually represent something called acne keloidalis. It is not cancer, but it does need treatment because it will only get worse if left untreated. See a dermatologist to confirm the diagnosis and obtain treatment.