DS#1 was around 10 months when we started trying, and around 15 months we I got pregnant. I had no problem with supply (I had always had an abundant supply) but he slowly self weaned, I think due to not liking the taste of my milk. He completely weaned by around my 6th or 7th month and he was around 20 months or so (my dates are off, but I can't really remember). There was an obvious drop in his nursing at around my 4 month mark when he dropped down to a couple nursings a day.
If I could go back and do it again I would wait until he was closer to two (which is my goal for nursing, at LEAST two years old). Which is why we won't start trying for #3 until #2 is at least 2 yo, probably later though.
I don't know what the percentage of moms who successfuly nurse through pregnancy, but I have two friends who have. One has 4 kids and was nursing two at a time until the youngest was around 6 months (so 6 years of nursing, 4 years of nursing two kids, 4 pregnancies total). Her children were around 4 months, 12 months and 14 months when she became pregnant with the next child.
My other friend has 7 live children and she has nursed through many of those pregnancies.