How long will iOS 12 receive security updates?

If so, how long can we expect to receive security updates? If we assume that the iPhone 5s won’t receive support after iOS 12 (its currently the oldest iPhone still receiving updates) that will make 5 years of support.

Does iOS 12 still get updates?

The iPhone 5s and iPhone 6 both run iOS 12, which was last updated by Apple in July 2020 – specifically the update was for devices that don’t support iOS 13. When iOS 14 launches it will run on all iPhones from iPhone 6s onwards.