Hello there,
AFAIR, there is no direct match for this ScreenOS feature in SRX but you can mimic this behaviour with following approach:
1/ write a policy with appropriate match criteria and action=permit. Let's call it policy #100
2/ write another policy with same match criteria and action=discard. Let's call it policy #200
3/ write a SLAX script which periodically reads "show security policies policy-name <name> detail" and takes session count from there
4/ should the count exceed the rising threshold, the SLAX script reorders policies in such way that policy #200 appears before policy #100.
5/ should the number of policies drop below falling threshold, script reorders policies back.
The only thing which is missing is session accounting per src IP but you can always write a separate policy per /32 src prefix.
Good luck!
Rgds
Alex