i would expect it depends on how many SPC which is installed in the srx5k chassis.
The scale numbers are above 20.000 (up to 50.000 on a fully specced SRX5800) concurrent users but I would not expect this with 1 SPC installed - so reach out to your local Partner SE to get a more specific scaling number.
SSL VPN on the newer releases and dynamic IPSEC VPN in older releases of Junos for SRX is a licensed feature only on certain models. So the limit is on the number of simultaneous connections that you have licensed for the device.
I do not see license options listed for the SRX 5k so this may still not be supported. Check with your reseller for the license maximum users per device model and their availability.
Steve Puluka BSEET - Juniper Ambassador IP Architect - DQE Communications Pittsburgh, PA (Metro Ethernet & ISP) http://puluka.com/home