It is a little bit behind where the experts would like to see her, but not worryingly so. The average 18 month old should ideally have around a dozen words but it doesn't have to mean something is wrong if they don't have that many, as children do tend to do things at their own pace and a lot of speech progress can be made very fast between 18 months and two years. (One of my nephews had no words at 18 months, but by his 2nd birthday was yapping away in three-word sentences).
My own 19 month old only has five clear words (Mama, Dada, bye, car, duck) but spends much of his time muttering and babbling to himself and I often catch word-like sounds coming from him, but it isn't clear what it is meant to be, or if it is meant to be anything at all!
At this age, understanding is still more important than speaking. If you ask her things does she understand without gestures to prompt her? eg. Bring me the ball Where's daddy? or offer her a familiar book and ask her to show you the pig/duck/car/whatever. This is a big favourite with most little kids in my family, including my son: www.amazon.co.uk/Babys-ABC-Sassy-Grosset-Dunlap/dp/044848207X/ref=sr_1_1?ie=UTF8&qid=1487366548&sr=8-1&keywords=Sassy+ABC&tag=mumsnetforum-21
If you don't have any concerns beyond her not having more words, I'd just have as many conversations with her as you possibly can, narrate everything you do with her (lets put your socks on your feet, and now lets brush your hair, here's your brush, etc) and hope she picks up more words soon.