According to "Forging Our Legacy: Canadian Citizenship and Immigration, 1900–1977" by Valerie Knowles, written for the Public Works and Government Services of Canada, the number of American draft-eligible immigrants to Canada during the Vietnam war years was between 30,000 to 40,000. Other estimates have it as low as 20,000 and as high as 100,000.

In 1977, President Jimmy Carter gave blanket amnesty to all Vietnam draft dodgers. According to Northern Passage: American Vietnam War Resisters in Canada by John Hagan, about half of all American draft dodgers living in Canada returned to the U.S. after that.

All told, that means that 10,000 at the low end and 50,000 at the high end, but most likely in the ballpark of around 20,000 draft dodgers stayed. They lived throughout the country. While I don't have yearly statistics, as of the 2011 Canadian census, there were about 730,000 Americans living in Canada, so the number of draft dodgers who came during the Johnson and Nixon administrations wasn't a whole lot more than a blip beyond the normal number of Americans who were immigrating to Canada at the time. While there are significant expatriate American communities in some major cities, notably Vancouver and Toronto, there's not an "American town" portion in any of them. Partly because there's little language barrier outside of Quebec, but also because many of these Americans also have connections with other communities (Chinese-Americans, for example) where they have cultural connections beyond the United States.