You are free to use your license key on client sites. However, we advise developers to encourage each client to purchase their own plan. This allows clients to have access to the support forums and receive ongoing updates. In our experience, each site having a unique license key and support membership benefits both the site owner and the developer. This protects you if you need to cancel your own license in the future, want to end a relationship with a client, or if one of your clients shares your key and it gets flagged for abuse.