The report I saw it in was https://www.thetimes.co.uk/article/...evealed-uk-home-to-23-000-jihadists-3zvn58mhq, obviously there's more to it than the headline number.
About 3,000 people from the total group are judged to pose a threat and are under investigation or active monitoring in 500 operations being run by police and intelligence services. The 20,000 others have featured in previous inquiries and are categorised as posing a “residual risk”.