I'm using 4 PC's at once right now, and I'm logged into 2 others...
I'm on a desktop, getting media from a headless server box while accessing the net through the smoothwall router box, and I just sent a print job off to the printer box which spools pint jobs for the printer.
Old PC's come in handy for the little jobs around the house, like the print sever, router, server, tho with the high availability of old PC's I tend to have PIII's even on the print spooler and router box these days. The print server used to be a p166 untill recently, but the really old PC's are dying off. The 'oldest' PC now is the dual P2-450 server box.
I'm also logged into the wife's PC getting some docs and doing some admin, plus there is a test machine I am working on via ssh...
So, easy enough to use more than one PC
But my comment was more about the total number of PC's laying around the house in various running and mostly non-running condition. They do seem to collect and I honestly don't know how many PC's there would be... I even have some 8088 bits laying around... (and z80 etc...)