I think the recent BASC article on the 243 mentioned that number.
Edit : just looked at the article - it mentions that 60,000 stalkers use the 243. There are about 150,000 FAC Holders. A reasonable proportion of those will be target shooters who don’t stalk, so 243 is used by a very significant proportion of deer stalkers. Appreciate that many FAC holders will have more than one centre fire rifle, equally many FAC holders will be non FAC holders.