This is the information form the Aruba 2930F / 2930M Advanced Traffic
Management Guide for Version 16.05 on page 96.
When the Switch is configured in a stack, the number of configurable MSTIs is limited to four.
However, when the Switch is configured as a standalone, a maximum of 16 MSTIs is supported.
I can't find this information in the guides for newer ArubaOS versions. But the changelogs for the newer ArubaOS versions don't contain any information about this.
Can anybody confirm that the limitation of 4 MSTIs for stacks officially does no longer exist in ArubaOS 16.06 and later versions?