With every C-section you have, your chance of serious complications—such as uterine rupture, placenta abnormalities, and postpartum hemorrhage—goes up.
These problems can be very dangerous for you and your baby. But it's hard to give a simple answer to this question: Every woman heals differently, and while one woman may be able to have three C-sections safely, another may be advised to stop after just one Cesarean because of significant scar tissue or poor healing.
But even one Cesarean section carries some risk. Your abdomen and uterus are being cut in order to remove your baby. And as with any surgery, there is the chance of bleeding, damage to other organs, blood clots, scar tissue, and adhesions. Subsequent C's are riskier than the first one, so the decision to have an initial one shouldn't be taken lightly. I've heard of women (even some physicians) who schedule a C-section for a first baby for the sake of convenience. This is major surgery, people! So unless it's medically necessary, you should take the vaginal route.
My advice: Discuss your desired family size with your doctor as soon as possible. She knows you best, and can give an answer that's based on your individual health history. There's recent evidence suggesting that some women can safely have a vaginal birth after Cesarean (VBAC). Ask your OB for her views on VBAC, and whether it could be a safe plan for babies number two, three, and four.