Hi,
I believe the following is as acurate as you can get "What are the risks associated with amniocentesis?
The risk of miscarriage due to amniocentesis is about 1% and this is the same as the risk from chorion villus sampling. If you were to miscarry due to the test, this would happen within the next five days."
From https://www.fetalmedicine.com/fmc/chorion/02-amniocentesis/
This centre is run by Professor Kypros Nicolaides who is pretty much one of the leaders in the field of fetal medicine.
Hope this helps.
It is always worth asking the consultant who does the amnio how many they do and what there own success rate is though.
xx