Yes, Stackable Premium is designed to be used for your own websites or for your client websites.
To make the most out of your Stackable License, you can opt for an unlimited license plan if you have a lot of clients.
If you're using Stackable for your client sites, you can choose to hide your account information in that site, more info in the link below.
Another alternative is to purchase Stackable Premium on your client's behalf, and then transfer the ownership of the license key to your client when you've deployed their website. The benefit here is you place the responsibility of renewal to your client. Find out more below.