Hello,
AFAIK, there is no way to do "indiscriminant severity match" in JUNOS event-policies as wildcards are not supported in [event-options policy <> events] stanza.
You have to do some research first, indentify events with your chosen severity threshold or above, and specify all of them in 1 or more event policies.
There could be only 20 policy instances being executed at any given time/concurrently, default is 15.
http://www.juniper.net/techpubs/en_US/junos14.2/topics/reference/configuration-statement/max-policies-edit-event-options.html
https://www.juniper.net/techpubs/en_US/junos13.3/topics/topic-map/junos-script-automation-script-memory.html
Chose Your events carefully or You could get no trap sent when there is a flurry of syslog messages.
HTH
Thanks
Alex