18 years old and telogen effluvium, is hair density going to go back to normal even if regrowth hairs are thin? (photo)

This requires an in-person evaluation. I suggest seeing a hair loss expert now to evaluate and go over your condition and provide you information about your best treatment options. There are great non-invasive options like prp/progesterone and/or hair transplantation. See an expert. Best, Dr. Emer

Regrowing hair is a good sign - but does not mean the TE is over. If one has not solved the underlying reason for the shed, the shedding will continue. Please keep close follow up with your dermatologist.

You have not supplied enough information to make any judgments. Are you are male or female?  You need to be examined by an expert doctor