I have read that children should have 20-50 words by 18 months, though I guess there is a big variation in this. 15 month old DS seems pretty average compared to other young toddlers we know, though there is no sign of Mama/Mummy yet 
At the moment he says about 11 recognisable words:
Dada
Ta
Yeah
No
Hiya
Bye-bye (buh-buh)
Boo (buh)
More (muh)
Again (geh)
Woof (wuh)
Quack (gak)
He is yet to have a developmental check with the HV so I'm assuming he's developing fine 