Hi dear friend
Every C2960S has up to 66 SPAN sessions, each SPAN session has up to 64 span destination ports and up to number of available ports on the switches for source span port.
When you use Stack, only the control panel protocols and technologies will be shared to increase the availability and performance.
SPAN is not related to this sharing, it implies you can use 66x2 = 132 SPAN sessions when you use stack between two C2960Ss.
Best Regards.
Eng. Maher Al-Jurf
CCIE R&S (CCIE#42758)
Senior Network and Security Engineer
Mob. +966-545958090
E-mail: maher7783@gmail.com