They can usually bring numbers in the low hundreds to their big protests, but the hard core appears to be much smaller. From a recent Environmental Review Tribunal hearing:
When asked, Palmer said he was a party to the appeal against the Enbridge Wind development in Kincardine in 2007.
"I was an appellant," said Palmer. "I was one of 37, three of which maintained full party status, while the rest were represented by one person."
"Many of those people became members of Wind Concerns Ontario?" asked Meuleman.
So the WCO core was about three dozen people. This is interesting in light of the public response to the Grand Valley Wind Farms Inc. project in East Luther Grand Valley, back in 2012:
A total of 89 comments were received in response to this Environmental Registry notice. Forty-eight responses were form letters which objected to allowing a private developer to harm species at risk and questioned how such authorizations can be justified under the ESA.
As I noted at the time, it looks like the WCO provided that form-letter. Looks like pretty much their entire membership recopied that letter.
So: several dozen of the committed who can rally a couple of hundred to their big events. That's the extent of it, it seems.