You are supposed to have 300mg or less of caffeine. That is 3-4 cups of coffee depending on the blend. I have a couple of cups of coffee in the morning and I don't think it has affected my weight loss. Plus it makes me happier.
As an FYI, you may be a little more sensitive to the caffeine while on the diet--so you can drink a little less and get the same effect! Or drink the same amount, and be very hyper