How many words should an 18 month old be able to say?

The number of words that an 18-month can speak varies from child to child. It also depends on whether the child is bilingual or not. Most toddlers at this age are expected to be able to string together two words to indicate what they want and why. Like milk-hungry, out-play, me-want etc. If your toddler has a vocabulary or about 25-50 odd words, he should be just fine for his age.