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

This can vary a lot child by child and will also be impacted by how many languages are being used in his surroundings. In Singapore a lot of children are exposed to more than just English. In our case, our son speaks a lot more English words than German words, since English seems to have a lot shorter, mostly one-syllable words. His vocabulary revolves around food names (bread, milk, water, banana, apple, etc), actions he does every day (come, play, breakfast, eat, drink, etc), and things he usually wants (out, down, up, etc). I'd say 20-50 words should be a reasonable amount. Be aware -again- this amount could be split into English and another language. Our son doesn't know all words in both languages. Here's a good article on child development including speech at the stage of 18-months. I hope it helps http://sg.theasianparent.com/toddler-development-18-months/4/