kdm and Ann are correct. I've been to 3 outdoor events (Bridgeville, Summit Point, and Mt Pleasant) so far this year, and none of them have had multi entry lanes, mostly due to the shortage of volunteers.
You definitely need 4 magazines per division, but having 5 is good insurance, especially if you use a Barney mag! I make it a point to recommend to new (or less experienced) shooters that they shoot 5 to Glock first then Glock M before the Plates so you can see where your pistol is printing at the various distances [make sure that you walk the targets as they're being scored so you can see the point of impact]. Personally, I believe that this approach increases your chances of taking down the Plates faster. I also recommend that shooters start with the lesser recoiling calibers and work up (i.e.: 22, 380, 9mm, 40 S&W, 45, then 10mm). I have seen very few 357 Sigs on the line in the past few years.
Usually, the fastest moving stage is the Plates. Glock M and 5 to Glock tend to get backed up faster because of the need to do a coordinated movement down range in order to score and paste targets. Helping paste targets helps speed up the stages.
Skippy, IIRC, you are from the north Jersey area. Are you making Jackson your first GSSF match?