I think you remember correctly. You need a DLL (CvGameCoreDLL.dll) that allows a higher civ count. So long as you don't use a mod that comes with its own DLL, you can just get a DLL from the CFC database that allows more civs and makes no other changes.
This one allows 34 civs; doesn't say that it's for BtS 3.19 (which, I assume, is what you have), but, since it was first uploaded in 2013, I suppose it is. This is the tightest limit I can find, others allow 50 or 100, which isn't ideal because you'll need to add a team and a player description to the scenario file (WBSave) for each civ allowed (and a high limit also slows the DLL down a little bit). Will need to use a text editor to add the extra civs to the WBSave. For the unused teams, just
should suffice, for the players
Code:
BeginPlayer
LeaderType=NONE
CivType=NONE
Team=18
EndPlayer
– with increasing team IDs. You could also take a look at the WBSave in
this mod, which uses 34 actual civs and a 40-civ DLL. As for loading your version of the DLL, you could put it in a folder "Assets" inside a folder "MyMod" (or something), place that in the Mods folder of your BtS installation and load the mod manually from the BtS opening menu or through a shortcut to Civ4BeyondSword.exe with a space and
mod=\MyMod
added to the Target field in the file properties of the shortcut. (Different procedure for Steam.) Your modified WBSave could also be part of the mod – inside a "PrivateMaps" folder at the same level as "Assets".