My understanding is that it is per phone line that has a plan that includes the 15 GB hotspot feature. On my account, we have 3 lines that have plans that include the hotspot, so each gets its own 15 GB limit.
On the Mix and Match Unlimited plans, the mobile hotspot being throttled after 15GB, is per line. The plans are per line, as well, therefore, the mobile hotspot usage is based on that line's usage only. You can learn more here: https://www.verizonwireless.com/plans/unlimited/. BrittanyC_VZW