I have a colony of 18 Dubs in a 6ft. 150G tank they are all about 1.5". I have had them since December 2010.
From what I've read on these boards, I could have up to 48 or so.
The question is, are these fish too old or too established to add more Dubs?
If they are only 1.5" and you really want to add more, it really shouldn't be a problem with Dubs, but, make sure that you change the layout inside the tank to help disrupt the pecking order. Also, Dubs get pretty large in captivity and 18 is likely a good number when they are full grown.