My son 18months ve only 8 teeths he is taking ca and vitamins syrup and he took a 5 ampoules of vit d 3 months ago and no more teeths appear again?

I'm concerned: Not about the timing of eruption of the next teeth (the baby canines), but about what you're putting into your child's body. Please immediately see 1. Pediatrician to evaluate the supplements your child is getting, and those he is not getting, and 2. Pediatric dental specialist to evaluate crania-facial growth and development, tooth eruption, oral hygiene, fluorides, and so much more. Go now.

Probably no concern: There is a "normal" time for teeth to come in. Not every one follows the same schedule. He is still within the normal "range" for eruption. I wouldn't be too concerned for another 6-12 months. My own daughter was 12 months before she got her first tooth.