I didn't hear all of Comander Shepherd's broadcast but I believe they're from her timeline, she's fought them before. No I don't know how many there are, I haven't had time to see if I can determine that through the Force.
From what I understand there's an air intake on the top of them, DO NOT allow yourself to be hit by the red beams they fire. The best thing now is to meet at Shepard's and armor up.