Running Junos 10.4, SRX650 supports 2k logical interfaces; a 'logical interface' includes a broad range of things: RETH is a logical interface, and each member of a RETH is a separate logical interface; subinterface is a logical interface; fxp0 and HA connections are logical interfaces; etc.
If you have a standalone 650 with no HA, MPLS, tunnels, etc then you can get close to 2k subinterfaces. As you enable other features use some sort of interface that number will decrease.
The amount of logical units depends on the type of physical interface and its encapsulation configuration. as to GE interface, if the vlan-tagging is not enabled, the amout is 1, if the vlan-tagging is enabled, the amount depends on the range of vland id, as to srx650, it is 0-4094. we can get the valid range of vland id by trying a invalid vlan id as below
root@jiaxin# set interfaces ge-0/0/2 unit 0 vlan-id 10000 error: Value 10000 is not within range (0..4094)