As I counted, Maximus's vocabulary has reach to 10 words since he touches 15 months...he started with car and now he picks up new words now and then.
The new words till today - bag, star
The accumulated words till 15 months - car, shoes (shoo), ball, bear, bus (baa), cat, clock (crok), milk, ma-mi, papa, biscuit ('kert), balloon (b'loon) etc...
It's not the quantity to count but it's more of the understanding and the use of the words in their daily communication. Maximus hardly use some words as he rather use his sign language or point to the item. I am sure sooner or later he will grasp more practical words and mostly through imitation.
It's a fun stage having to listen to them speaking... from single words to multiple words to sentences.