I have actually lost track of how many companies I am signed up with, but by now it is well over 200. Most of them are useless for my particular area or have become useless either because of the kinds of shops, their notions of an appropriate fee or they have lost their client(s) in my area. You will not know that they are useless to you until you have monitored them for a while.
There is a group of 30 companies that are my "useful" companies at this point. I try to check their job boards at least every couple of days and with time you will figure out when they are most likely to post the jobs you are interested in. I keep them all in one bookmark folder with my Firefox browser. Then I have a bookmark folder of "new" companies, these being places I have signed up with during the past few months. These also I check every few days. If, over several months, I see nothing on their sites, their link will just be moved to one of the "MSP" bookmark folders so I can access them every couple of months to see if anything seems to have changed but I am not checking them often.
Once you have done several jobs with a company they are more likely to email you jobs that are posted--some of them send the emails ad nauseum--but many of these emails are to all possible shoppers and need to be acted on quickly or the reasonable jobs will be gone. More selective emails come after you have worked with a company more. My experience is that job boards are more useful to find who is shopping your area that you are not signed up with than for actual jobs because there are too many shoppers watching the boards.