This KB describe how you can calculate the number of ARP entries learned when you have a Redundant Ethernet (reth) interface configured on the SRX.

The PFE has lower limitation than RE, and it creates more NH as well compared to on RE, on the RE the limit is 100000 and in the PFE, the limit is 64000 (for standalone system) "ARP Table(Static)"

If you're configuring a Redundant Ethernet (reth) interface with more than one physical interface from each node of a cluster, each ARP entry learned on that reth interface will consume an entry for each of the reth member ports (eg: if you have 2 physical interfaces configured under the reth, that will make the ARP entries learned there consume double the resources).

Also, the number of arp entries can be checked from cli by using the command "show arp no-resolve | count " and "show software-nh index"