Forum OpenACS Q&A: Response to Better Performance: Multiple AOLserver(s) vs. One nsvhr'd AOLserver?
Regardless, perhaps my brain is fuzzy at this moment, but your question doesn't seem to parse. If you want to host n ACS's (version 3 series) on one box, you need need at least n AOLserver processes. What nsvhr will do is multiplex requests to your system so that all n ACS's can seemingly share port 80 (or whichever port you want them to share.) so that's n+1 processes. Nsvhr does this by copying portions of the request.
It is almost certainly true that n+1 processes and a copy is less efficient that n processes, but it does give you the benefit of sharing port 80.
I believe I have read that in ACS 4, we should be able to host multiple ACS sites in just one AOLserver process. (The reason you cannot now is that the names of database pools are hardcoded into the code.)
Even if ACS 4 lets you use just one AOLserver process, it is quite likely that folks who want to run different versions of ACS at the same time will still benefit from nsvhr if only to keep eliminate potential ACS library conflicts.