https://www.gov.uk/right-to-buy-buyi...-home/overview

The discount you get might be reduced if you’ve used Right to Buy in the past.

You’ll usually have to repay some or all your discount if you sell your home within 5 years.

https://www.gov.uk/right-to-buy-buyi...-home/overview

they have not bought them from the council its impossible to buy that many, they've bought them from people who used the right to buy and bought them from the council.