When I first started back in 2003, I signed up for anything/everything. But as far as how many I actually do shops for regularly, it's less than 10. Just because you can sign up for something doesn't mean you always should. I guess it doesn't "hurt" to sign up in the off-chance they land a client, but some of these MSC seem to focus on a region and even though I signed up, I never landed anything with them.
The huge companies get the national chains, so focus on the bigger companies like Intellishop, Market Force, Ipsos to begin with. Then you can add from there.
One thing that would be nice to see, is if people would regularly post regions they are seeing things as some things might not be nationally shopped or available.