I don't think there's anything canon, but you can make a few assumptions.
I think the best assumption to make is that the most common species are those enslaved by the Hutts (Vodrans, Klatooinians, Nikto) and similar races used by the Hutts as mercenaries (Weequay).
My logic: for the slaves, I'm thinking that the Hutts would do their best to ensure a constant stream of slaves for every generation. They probably breed their slave races for many traits, and numbers would be one of them.
25,000 years of slavery should ensure a large number of Vodrans, Klatooinians, and Niktos in the Galaxy.
Twi'leks would apply similarly. Twi'lek slavery is common (not just for the women), and so a slave population might be bred to maximize their numbers.
The same logic might apply to the hired races, but from a free will point of view. Each family is well off working for the Hutts, so they make sure there's enough in the next generation to serve further.
But that's just a thought.
"I must not fear. Fear is the mind-killer. Fear is the little-death that brings total obliteration. I will face my fear. I will permit it to pass over me and through me. And when it has gone past I will turn the inner eye to see its path. Where the fear has gone there will be nothing. Only I will remain."
-Bene Gesserit Litany Against Fear