The number of attendees at any given political event always seems to be in contention (for obvious reasons). I believe that there were at least 10,000 people at the Kerry rally; Grand Rapids leans very democrat - and the unions were really rattling cages to turn out their members (even those from out-of-town). Though it's only anecdotal, I stood in line with many union folks.
The spokesperson for the city estimated the crowd at 9,000-10,000:
http://www.thehollandsentinel.net/stories/080304/loc_080304001.shtml
The Grand Rapids Free Press estimated the crowd for the Kerry Rally to be 10,000:
The State News has 20,000:
http://www.statenews.com/article.phtml?pk=24822
Wood TV has pictures of the event showing how packed Calder Plaza was:
http://www.woodtv.com/Global/story.asp?S=2110785&nav=0RcePQ78
If Chief Dolan counted only 5,000 - it must have been during Teresa Heinz Kerry's rambling, long-winded speech when everybody went to go get a hot dog and lemonade.