16 months- only one solid word

Our little girl is 16 months old and only has 1 consistently solid word. She will say “hi” all day long. She does say it at the appropriate time and she will often wave when saying it. She does babble all the time and has recently started saying “taa” and I know she is trying to say tots (like the show).

Should I be worried? What can I do to help her expand her vocabulary?