Instances are the amount of time you would like to spawn a session of your application Example:
On a 12 slot(core) machine running Maya if you set
"Instances" to 4
"Reservations" to host.processors=3
Qube would open 4 "Instances" of Maya on the worker simultaneously
if you set
"Instances" to 1
"Reservations" to "host.processors=1+"
Qube would open 1 "Instance" of Maya on the worker
more on Instances & Reservations