I was taking a university class about language acquisition when my AS son was 18 months old. We didn't know he was AS at the time, but it was fascinating watching him learn language. From 12-18 months, he was continuously learning new words, but he only said them one at a time.
In that class, I learned that at 18 months, the average kid has about 50 words in their working vocabulary, like you said. Not necessarily pronounced correctly, since they haven't worked out how to make the sounds perfectly, but with the correct usage. My kid was different. He had about 350 words. I know, because I was keeping a list, since it was so fascinating to me. His words were things like, "statue", "porthole", chimney", "Volkswagon", "fire engine", "wheel". He did have words for mom and dad, but they weren't among the first. The first was "fish", because of the fish on our shower curtain.
He never took naps, but I used to put him in his crib for an hour each day, with a bunch of board books, for our "quiet time". I would hear him in there, talking. He had babyish pronunciations of a lot of words, but if I spell the words normally to make it easy to understand, his talking would go something like this:
"Wheel.... Brrrrr. Round. Wheel. Fire engine. Fire. Hot! Hot! Hot! Ow! Fire engine. Hose. Water.... Bathtub!" It was fascinating to listen to him at those times. He would been looking at a book with a picture of a wheel or a fire engine, and then been having a train of thought, leading to different ideas. As far as I can tell, his language development was unusual.
I don't know when I began to talk, but I've been told many times that my first word was "no." Apparently it was used to express an opinion (as in answer to a "do you want..." or "do you like..." kind of question.