You might be able to get some info from Furfly given that they are doing OpenACS hosting with multiple instances per box, but in general, pushing hardware limits with OpenACS is still a new field :) I have about 5 instances running on a dual Pentium II-400 with 256 megs of RAM, and I'm pushing things a bit on that.
In general, if your sites are low traffic, you want to worry mostly about how much RAM you have. Each OpenACS instance requires one AOLserver, plus usually 10 DB connections. Each DB connection is a PG backend process. I would suggest having at least 30-40M of RAM per OpenACS installation, and preferably 50M or more if you can spare it. The second important aspect is the speed of your drives. Having Ultra SCSI drives will do *wonders* for you, simply because of all the disk writing that any interesting OpenACS site will do. The third aspect is processing power. Count up the # of hits you're getting on all sites combined. My estimation is that a single Pentium 800 box with Ultra SCSI drives can take about 1 million OpenACS-like hits per day before it maxes out. This figure should be confirmed in a more scientific way, but that's the trend I'm seeing.
Thus, again, the big determining factor is RAM, and to some extent disk speed.
