Do you mean using the old-fashioned rhythm method where you look at a calendar to tell you when you're "safe" or do you mean using the scientific sympto-thermal method where you using your body's fertility signs and your temperature every morning to tell you what days are "safe" ?? (also called NFP or FAM)
Also, do you mean how many people used NFP/FAM to get pregnant when they were trying to conceive? Or how many had an "oops?" Oh yeah, and then one more question - do you mean did they have an "oops" when they DTD knowing that they might realistically have an oops, but did it anyway, or an "oops" when they were following all the rules?
Just asking. Your question is a bit ambiguous!

I've used NFP for 3 years to avoid having a child, and used it for one month to try and conceive (successfully). I do not use the calender method and it would completely have failed for me within a month or two as I am very fertile and have irregular cycles. But clearly NFP works great, as long as we follow the rules, which we do since we have reasons to plan our family precisely.