My niece is 20 months old and doesn't have any words at all. I know all DC are different but I'm sure mine were starting to speak in 2-3 word sentences by this age. My sister thinks she'll just get it one day and hasn't mentioned, nor does she plan to mention, it to her GP or HV. I agree that she will very probably just start talking a bit later but I do think it's worth mentioning to a professional because waiting lists for SALT are so long around here so it does no harm to already be on it.
I've broached it in a non bossy or judgy way but DSIS thinks it's common for children of this age to not be speaking yet.