Hi,
its 2M max for high end mx such mx480, mx960 and above.
It's editable from junos 16.1 onwards:
re0# set system arp-system-cache-limit ?
Possible completions:
<arp-system-cache-limit> Set max system cache size for ARP nexthops (200..2000000)
{master}[edit]
Please review the following link for details:
https://www.juniper.net/documentation/en_US/junos/topics/reference/configuration-statement/arp-system-cache-limit-edit-system.html
The default behavior of ARP is to remove the cache entry if it is not used within a certain period of time, and not to allow the cache to grow too large. You can also manage the number of ARP cache next-hop entries by configuring a limit to the maximum number of next hops that can be created.
Options
number—Indicates the maximum number of routes to be held in the ARP cache.
Range: 1 through 2,000,000
Default: (ACX Series routers, EX2200, EX2200-C, EX3200, and EX3300 switches, SRX Series services gateways) 20,000
(EX4200, EX4300, EX4500, EX4550, and EX6210 switches, MX Series routers) 75,000
(Other platforms) 100,000