There's no magic number...everyone responds differently and for some 3 or 4 is sufficient. I know it can get tricky coordinating the shakes with your usual routine...especially as you take more shakes a day. I would suggest that you develop a schedule and stick as closely to it as possible...this seems to be the easiest way to get it all in.
I'm not real clear about your elimination...you say you're not blocked up but you haven't passed two full days of meals and shakes? If you haven't had a bowel movement with all that...that sounds pretty blocked up to me! Do you mean that your bowels have moved, but you haven't seen signs of the shake? Ideally your transit time should be 18-24 hours. Rather than using stimulant laxatives...give yourself an enema or get a colonic to keep things moving.