How much tax do you pay when you sell a rental property?

That depends on a number of various factors. Each situation is different, you should engage an accountant. The BLA have qualified accountants who are a BLA service provider for members: https://thebla.co.uk/tax-support/?swcfpc=1

Alternatively you can read this blog which i found useful: https://thebla.co.uk/tax-tips-for-uk-landlords-2020/?swcfpc=1 you have access to the BLA partnership SimpleTax which is free to use: https://thebla.co.uk/tax-support/?swcfpc=1