Can I use Shutter on multiple devices without purchasing another subscription?

Yes, as long as you are logged into the same Apple ID account on each device, you can install Shutter on multiple devices.

To use Shutter on another iPhone or iPad, tap “Restore Purchase” on the subscription page in the app. As long as you are signed in to an Apple Account with an active subscription or a lifetime license, Shutter will be activated on your new device.

However, remember that there are different versions of Shutter for different camera brands, so you can only use one subscription for one brand. Read more here.