Quote Originally Posted by Ailurus View Post
One Dreen not working for Klaus is confirmed in-comic. There was a Dreen released from the vaults in Saint Spazc who later appears to Gil and Bang confirms it's not one that works for Klaus.

There were three Dreen seen in the Robur Heterodyne flashback. Back at the very beginning of the strip, there are two Dreen seen fighting the Wasps on Castle Wulfenbach and when Gil is talking with Thotep he confirms that two Dreen met and worked for his father.

So I think it's safe to say that when Klaus returned from Skifander one Dreen stayed with Robur's device in the Corbettite Vault while the other two met with and worked with Klaus. After the battle at Saint Spazc, the third Dreen met up with Gil and rejoined the other two, and now we have a group of 3 Dreen traveling around.
There are more Dreen than that - When talking to Kjarl, Gil says "the ones in their crypts" - we only see one (or possibly see two, but falsly assume that they are the same Dreen), so there's at least two with the Corbettites.