The number of victims often mentioned is 1’500. However, that was about the known number of victims in Rotherham alone during a certain period.

In reality the number of victims accumulated over the whole country up to this article in 2014, is believed to be as high as 600’000:

https://www.dailystar.co.uk/news/latest-news/405477/Top-cop-warns-600-000-children-may-have-been-victims-of-child-sex-abuse

More background e.g. here:

http://www.pmclauth.com/articles/grooming-gang-statistics

http://pmclauth.com/sentenced/Grooming-Gang-Statistics/Gangs-Jailed