You don't need to purchase multiple licenses for Windows and macOS. Which means, if you buy a product key for Windows, it will also work on macOS and vice versa.
With one purchase, you can install Cold Turkey products on all computers that you personally use. While there are no hard limits, one license for every 3 computers is typical.
For products that don't use a product key (Writer and Micromanager), please purchase the product for the OS you use most often. Then make an automated request for new download links from the buttons on these pages to receive download links for the other OS: