Hi Steve and Mannan,
NCP Remote access is supported on entire SRX range (300 series, 550HM, 1500, 4100/4200 and 5000 series) since 15.1X49-D100 (ref: https://www.juniper.net/documentation/en_US/junos/information-products/topic-collections/release-notes/15.1x49-d100/topic-120471.html#jd0e434)
On SRX300 and SRX320 it can only be done as a ipsec client but from SRX340 and higher it can be done via ssl vpn.
Licensing is the same across all platforms but obvious the scale is different:
There is no scaling numbers for SSL VPN per SPC so you will need to reach out to your preferred Juniper partner/reseller to have the ask Juniper internally.