Well, there are two parts to this question. Part one is: does having more children make it more likely that I'll have complications. I'm going to let pamamidwife or drjen field that one, although I think the answer is no. In obstetric circles I know being a "grand multip" (having more than 5 births) can be considered a risk factor, but so can being "elderly" (over 35!). It's just bogus, IMO. I live in Utah, so we see our share of "grand multips" and by and large, they do great! Most midwives say, "they've done it so many times, they know just what to do."
Part two is a statistical question, not a midwifery question. If there's a 1 in 5 chance of having, for example, a Cesarean, and I have 5 children, am I guaranteed a Cesarean at some point. Statistically, absolutely not. It might seem that way on the surface, but of course we know that's not true, because the truth is, these things are hugely influenced by many factors, it isn't just luck of the draw. For example, if you're seeing a midwife for care, your Cesarean "risk" drops to more like 5-15%, depending on the midwife. And if you've had a Cesarean before, you're statistically (because of encouragement by your care provider and society in general) more likely to have a repeat Cesarean, not because it's neccesarily called for, but because you had one before. Not fair, yes. The way it is, yes. It's like if you have 4 kids, and the marijuana use rate among teenagers in your community is 1 in 4, does that mean one of your kids is using? Nope. None of them may. Or all of them may. It's spread out across the whole community, so your family, and the family next door, and the family on the other side might have no drug users at all, but the family at the end of the street, all 3 of their kids might use. Does family environment affect kids' decision to use illegal drugs? Definitely. Can it still happen even in the best of families? For sure. But it's highly affected by what we do or don't do. Most things that "happen" in pregnancy and birth you can affect by your behavior, and therefore, your "risk" isn't related to how many kids you have as much as what you do.
That said, there are things that strike randomly. Birth defects is an example. Beyond the normal things almost everyone does (avoiding teratogens, decent nutrition, etc.) there's not much you can do to affect your risk of having a baby born with genetic anomolies. And if you have 10 children, then yes, you're rolling the dice 10 times. The good news is, these are usually very uncommon occurances, 1 in thousands. If you're having thousands of children, well, I just don't know what to tell you. I'm certainly not going to give you
For most things, there is an element that is random, uncontrollable, and an element that is affected by what you do. For many things, like Cesareans, pre-eclampsia, arrested labor, etc., popular belief is that they just "happen" randomly, when really there are many things you can do to affect it. Although, even with the best choices and care, anything can happen. So when someone tells you a "risk" of having lots of children, ask them if that's a random chance, or if there are things you can do to affect what happens. Usually it's the later and people just don't like taking responsibility for themselves.
(whew! long post!)