Whenever traffic passes through your firewall from one zone to another, a session is created. This graph is showing the number of active sessions. On the high end SRX, there must be one Services Processing Card per node and it looks like you have two SPCs in your chassis and since you have them in a cluster and two nodes in a cluster need the exact same hardware configuration, you have a total of 4 SPCs.
Actually I am aware of this SPC thing but the issue was these numbers 22, 23 and 6 and 7? what are these numbers? Also please confirm how these sessions are shared on these SPCs. As far as I know, if SRX is working in cluster the total no of sessions are shared amongst each node (need your confirmation). Now in our case since we have 2 SPCs on each node, are these working in load sharing basis? If so, the graph should show equal no of sessions on each SPC. Appreciate your confirmation on this.
When you activate CLustering on SRX, there is a change in naming convention. For example on SRX3400 we have 8 CFM slots (on which we insert IOC/SPC/NPC), on node 0 they are named as slot 0-7 and in node 1 they are named as slot 8-15
Below URL explains the naming convention in clusters.