The CLI shows 2 licenses included with the box:
root@srx300> show system license
License usage:
Licenses Licenses Licenses Expiry
Feature name used installed needed
dynamic-vpn 0 2 0 permanent
I expect the license scheme used on the SRX300 is the same as the legacy branch series (SRX-RAC-X-LTU where X is 5,10,25,50,100,150,250) but please check with your reseller/partner to avoid issues.
The SRX-RAC- licenses are still present in the latest pricelist from Juniper.