I can't answer the performance characteristics completely, but I think you have a couple of options (or questions to ask).
A. You can install, I gather, 20 subcommunities within one OpenACS instance.
B. You can install 20 ACS installations all using the same instance of AOLserver.
C. You can install 20 ACS installations, each using a different AOLserver instance, and use nsvhr or Apache or SQUID to multiplex port 80.
Will all these guys be on exactly the same version of OpenACS and be sharing the same customized modules? If so, that's solution A or B.
Or will these folks need different tcl libraries? That's C (or B using AOLserver 4).
Will all the sites be upgrating/evolving at the same rates? A or B.
Do you want to guarantee db isolation? C.
Would you prefer that all your communities die at the same time should an AOLserver bug crop up? A or B.
Or would you prefer that one community die with one AOLserver at a time? C.
When the 21st dude shows up, are you willing to stop all the other 20 websites to bring up the 21st? A or B. Do you want the other 20 to be relatively independent? C.
When the 26th site comes along, and you find your 2G box crawling along, would you prefer having had a solution all along that lets you clone the box and move sites 13-26 to the new box, or would you prefer just starting with a new box and site 26, giving you one box with 25 sites and one box with site 26? That's C, or (A or B) depending on your choices.
