A 2014 report by Gwynedd Archaeological Trust (GAT) entitled "Medieval and Post-Medieval Agricultural Features in North-West Wales" was used as the main source for the locations of the sheepfolds, as well as data from Ordnance Survey maps. The GAT report drew together information from the Royal Commission for Ancient and Historical Monuments in Wales (RCAHMW) 1987-9 Uplands Initiative as well as from a dissertation done in 1998 by a Manchester University undergraduate, Fiona Johnson, on multicellular folds. The GAT report, and dissertation, can be found here.
These records show at least 3500 sheepfolds in north-west Wales, but GAT themselves admit this is probably an underestimation. There is a distinct concentration of the folds in the north of the area, particularly around the Carneddau mountains and the reasons for this are explored in the section on the history of the sheepfolds.
A map of the sites noted above is shown below, reproduced by kind permission of GAT.
You can see a map with details of all the sites I have visited in the MAP section.