For open-iscsi you would set
# To control how many commands the session will queue set
# node.session.cmds_max to an integer between 2 and 2048 that is also
# a power of 2. The default is 128.
node.session.cmds_max = 128
# To control the device's queue depth set node.session.queue_depth
# to a value between 1 and 128. The default is 32.
node.session.queue_depth = 32