I used to vary it when I 8-12 tabled them a few years ago. If I was busting out of quite a few, I would fire more up immediately, but if I was still in a few after 20 or 30 mins then I'd usually concentrate more on them, since I always had trouble keeping up when it got 4handed and less and I had lots of tables going. Its just preference though, as I might be able to keep up w/ 12-16 nowadays no matter what point of the sng they are at.
But the best is to play 8 or so until they are about to get shorthanded then open another 8 up. As you don't want all your SNGs going shorthanded at the same time. Worked well for me, much better system than just loading up 16 to start with cos it just gets too hectic at the end.