A few weeks ago, my 20 month old had about the same vocab you describe, then it changed literally overnight. She now repeats everything and puts words together. Today she said, look. mum. cow. moooooo. i can't believe how quickly her vocab has grown.

I am pretty sure the average number of words at 18 months is 10. Given that it is an average, there are going to be some kids wioth more and some with less. Your little girl sounds perfectly normal.