I’m guessing your referencing the MAXACT of the pool each subsystem is assigned to? You can view that by WRKSYSSTS and change the value manually. If you set this value too low, jobs will go in an INEL state.
The other activity level within a sbs is how many jobs are allowed to submit per jobq. DSPSBSD option 6 to view your current setup. You can change the Max Active value on the fly with CHGJOBQE.
I believe the activity level is used when you allocate memory to a subsystem.
You can see the value when you look at the pool definitions.
MAXACT is used in two ways.
First is how many jobs can be active at anyone time in the susbsystem.
Second is how many jobs per priority can be active at anyone time.
You can setup QBATCH to have nomax, but only allow n number of jobs per priority level.