I had 2 c sections. They were less than 2 years apart. First one was an emergency, the second one was my choice. My dr. made me sign for which I wanted the VBAC of another c section. The vbac paper said that there is more harm to the baby than to me, and the c section was the oppisite, more harm to me and to the baby. So, I wouldn't have been able to live with myself if I had chosen a vbac and something had happened to the baby. I also think if they are less than 18 months apart they want you to have another c seciton and not try a vbac.
Also, once you have a c section they can't induce you or give you anything to help the contractions along because there is a chance that your uterus would go into a contraction and not stop. So, since I was 10 days over with my first, and they had to induce me, and then I ended up with the emergency c section, it was a pretty easy choice the next time around for me. But go figure with the second one, I was 3 weeks early!
My dr. doesn't recommend any more than 3 c sections, but that may be different from dr. to dr.