I looked some figures up on this a year or two for a school project. These figures are from a study totalling names from the 6 Wales telephone directories.
Here are the most common 40 surnames:
- Jones - 60.57 per thousand
- Williams - 37.81 per thousand
- Davies - 36.76 per thousand
- Evans - 24.66 per thousand
- Thomas - 22.97 per thousand
- Roberts - 17.14 per thousand
- Hughes - 13.38 per thousand
- Lewis - 12.96 per thousand
- Morgan - 11.7 per thousand
- Griffiths - 10.9 per thousand
- Edwards - 10.33 per thousand
- Owen - 8.85 per thousand
- James - 7.66 per thousand
- Morris - 7.5 per thousand
- Price - 7.5 per thousand
- Rees - 7.3 per thousand
- Phillips - 6.34 per thousand
- Jenkins - 6.23 per thousand
- Harris - 5.79 per thousand
- Lloyd - 5.64 per thousand
- Richards - 5.51 per thousand
- Powell - 4.72 per thousand
- Parry - 4.49 per thousand
- John - 3.65 per thousand
- Watkins - 3.39 per thousand
- Howells - 3.25 per thousand
- Pritchard - 3.14 per thousand
- Rogers - 2.71 per thousand
- Matthews - 2.37 per thousand
- Rowlands - 2.35 per thousand
- Humphreys - 2.22 per thousand
- Pugh - 2.19 per thousand
- Ellis - 2.17 per thousand
- Bowen - 2.17 per thousand
- Hopkins - 2.14 per thousand
- Martin - 1.91 per thousand
- Bennett - 1.86 per thousand
- Bevan - 1.69 per thousand
- Pearse - 1.65 per thousand
- Adams - 1.61 per thousand
These top 40 surnames only cover 38% of the welsh population, but this could easily be explained by varying numbers of people per telephone, studies taken from different eras, et cetera.