If I buy a SunShine license, can I use it on multiple phones?

No - one license is good for one device. You can reuse SunShine on that device, if necessary, we however NEVER recommending setting your device S-ON and we can not be held accountable if SunShine is incompatible with your device after the first s-off. We did try a license replacement program early on, however the majority of replacement requests turned out to be fraudulent forcing us to stop replacing licenses.