According to CBC News:

The Americans say that as many as 500 Canadian citizens may now be fighting for ISIS in Syria and Iraq. If that number would be true, the Canadian situation would be disproportionate compared to almost every country in the world.

A CBC reporter estimates the number as being somewhere between 200 and 250.

The Canadian government estimates at 130 the number of Canadian citizens fighting for ISIS in the Middle East.

CBC News (September 3, 2014): What’s behind homegrown extremism?
Shorter clip on YouTube (1min 26 sec): http://youtu.be/oDsui-fL_lM

Back in 2013, CSIS Director and now Deputy Minister of National Defence for Canada Richard Fadden had warned that “Canadians are involved in ‘every single case’ of al-Qaeda-affiliated groups” worldwide.

Tom Parry (CBC News – February 11, 2013): The head of CSIS warns that Canadians involved in ‘every single case’ of al-Qaeda-affiliated groups and that these groups have mentioned Canada as a possible target (Web Archive – Archive.today)