It’ll be useful to me if there were two auxiliary color buffers to play with. How many auxiliary color buffers might be expected? I guess this is a “it depends” question but a rough feel would be appreciated.
Thanks for any advice!
On most hardware, zero. 
Some of the newer, high-end ATi and nVidia cards actually have hardware support though (finally). You should be able to look up the specs on Delphi3d. A quick looks suggets 4 would be common ( http://www.delphi3d.net/hardware/viewreport.php?report=1341 )
We have a 9500 Pro here, it supports 2. I guess that means any card with ARB/ATI_draw_buffers support should have at least two available.
Thanks for the feedback - sounds like it’s not something I can necessarily rely on!