Do you have to buy paint tool sai again to use on another computer?

Yes, if you want to use Paint Tool SAI on another computer, you typically need to purchase a new license unless you have access to your original license key. The software is not tied to your account, so transferring it between computers requires you to install it and enter your license key. It's advisable to check the specific licensing terms on the official website for any updates or policies regarding multiple installations.