Due to the ethics involved in doing research, especially sex. It is very difficult to find studies that show estimates of sexual practices, especially cuckolding. Finding statistics on threesomes, swinging, and polyamory because each practice is common whereas cuckolding is less common.
Nonetheless, as I was doing some searching for cuckolding statistics I did come across a few good cuckolding articles on the Examiner. This link will take you from this site to the Examiner, which in no way associated with this site, and provide you with a list of articles. In the list there is one article of interest that talks about the rise of cuckolding but unfortunately does not provide concrete numbers. Also I do find the Examiner’s site lacking on specific information but is a good source for general information. Likewise Psychology Today has an article about the rise of cuckolding. Again it does not provide a concrete estimates on the practice but discusses it rise in popularity.