I have made a long list of 80+ agents/agencies from the 'Writers' and Artists' Yearbook', and the 'Guide to Literary Agents'. I will check out their websites and see if their requirements match their listings, because some of their requirements may have changed. Then I will check their submissions policy. I have already written a synopsis and covering letter, but I am still polishing my manuscript. When I'm ready I will send out 10 submissions to test he water.
It depends... Some will recommend that you send out to more than just them as the process can take a long time. Some however will mention in their submission guidelines that they will not accept simultaneous submissions.