I don't see any such IP limitation on SRX, at least there is no document stating that; however from the lab test we can add 300 IPs to physical interface but if you create number of VLANs and assign IP to each VLAN will hit number of VLANs per model limit.
"Maximum Number of IP Addresses in Trusted Interfaces Unrestricted"
This parameter was once a popular licensing limitation with firewalls many years ago. They would limit the number of host ip addresses that would be able to cross a firewall from the protected network. Typically a 10 or 25 host license. They kept track of the number of active ip addresses and then prevented access across the firewall when the count was full. These devices then sold at a reduced price for very small branch offices.
Netsceen marketed their firewalls as unlimited ip address in order to contrast with this old licensing model and the language has just carried over as the years march on and it no longer has any relevance. I don't think anyone in the firewall business still cripples the devices with host ip limits.
So to answer your question, the SRX is the same as the SSG for this parameter. There are no limits to the number of hosts you can place behind the firewall.
Steve Puluka BSEET - Juniper Ambassador IP Architect - DQE Communications Pittsburgh, PA (Metro Ethernet & ISP) http://puluka.com/home