Can I use the same plugin activation key for multiple websites I own?

It solely depends on the license type you have purchased. If your plugin license is on a ‘single site’ type, you cannot do so. However, for a 5-sites license, you can add the same activation key for up to 5 websites. Similarly, for the unlimited site license, you can use it for up to 25 websites. So, carefully choose your license type as per your needs.