Can you let me know how many different reasons are there for hair loss or thinning hair?

Many: The evaluation of hair loss is an involved process because there are so many possibilities. General classifications are scarring versus non-scarring. Scarring alopecia is an inflammatory problem (e.g. Allergy or infection). Non scarring alopecia can be caused by medications, poisons, heredity, medical conditions and severe hair care practices. See your dermatologist.

Too many to count: Mostly, it is probably simple genetic programming that you have inherited. If this is a recent change, however, you may need to look into hypothyroidism, anemia, potassium imbalance, diabetes, or other metabolic disorders. Talk to your fp about about testosterone levels and other blood testing to help direct therapy.