I don't know that there is a way to define that within the expression itself but DLPe can define the threshold for triggering an incident inside of the advanced pattern definition. Since I don't know which version you are using I couldn't say where you should be looking but there is a 'Threshold' section in both 9.3 and 9.4/10 when configuring advanced patterns.
Using 9.4 -- there is not a setting in the advance pattern definition for setting an "occurs X times" clause. I only see this option in the proximity def. No longer have any issues this has been figured out.