chromium has a switch --renderer-process-limit=n
I've tried it. It does cut down on the number of chrome.exe instances. Only thing is I don't know how to restore the default. One one of the pages it mentions it being set to 42. So I tried running it with that. Seems to put it back to "normal" behavior.
Without the switch the 23.x snapshot I'm using is running 11 instances. I have 3 tabs open adn 5 extensions enabled. The weird thing is, if I set it to something line 4, the number of instances is reduced. But I don't really see any difference with page behavior.
I just restarted chromium with --renderer-process-limit=4 and with only one tab open and 5 enabled extensions, I have 6 instances of chrome.exe. I probably won't notice any difference unless I'm using multi-media stuff.