The girls were aged between 11 and 18. There are about 3.5 million girls in the UK in that age range, an eighteenth of the population as a whole. Assuming that Rotherham (a city of about 250,000) reflects this, there are some 13,900 girls in that age range at any time from that city. The abuse took place over a 16-year period, so we can slightly more than double that, to (say) 28,000, to get the total figure for girls who were in that age range in Rotherham during the period covered by the report.
If the report's findings have been correctly reported, this means that 1 in 20 girls in the city were abused. That's one or two for every classroom, for at least a generation. My maths and/or facts may be wrong, of course - I welcome corrections.
The head of children's services at the time, a master of the passive voice, "regrets that more wasn't done at the time."