I'm not sure on the lore, but in game there is a second Dashade (or something that looks exactly like Khem) on Voss in the Nightmare Lands. Republic side, at least.
That thing you meet on Voss is not really a Dashade, it just uses the same model for some reason.
Speaking about other Dashades... well, we know that their home planet was destroyed, as well as the majority of its population, but a very small number of them is still scattered across the galaxy.
If i'm not mistaken, in-game we can meet one more of them when doing a special Khem Val's quest.
Also, Khem is definitely not the last Dashade. The remnants of this race have managed to survive for thousands of years and are still present during the Rebellion era (where the classic episodes take place). Here is one of them: