It's implied, but we only see two Companions total, Inara and the former companion running the Heart of Gold; and she offers Mal a boy when he turns the girls down, so yes, they're there.

You know what I'm like, I don't tend to go into huge amounts of detail, so I could dig around the sites but essentially it's implied there are both, and Inara is shown turning down potential clients a few times (and has a few female clients at times).

See, you now have another reason to watch, you can critique the portrayal of the companions ;-)