In the filters page, where you can set up the rules, you need to add an extra line to your filter -- click the "+" half of the +/- icon at the right side.
Then on the new line that pops up
"and" "Monitor Name" "equal to" "P1"
This will give you just events from P1. You could also do it as "not equal to" "NOTP" instead.
Test with the "submit" button to see you are getting the events matched correctly.
I don't know how to do, for example "Date is today" AND ( "Monitor name equals P1" OR "Monitor name equals P2" OR "Monitor name equals P3" ) correctly, because that *needs* brackets around all the ORs to gather them up to make sure it does the right thing.
You may end up with more than one rule doing similar things, so one rule for P1, one P2 etc.
Have a play with those options.
[edit: I do know how to do a load of OR's ... look at "and" "Monitor Name"
"in set" and then put "P1,P2,P3,P4" as the set of cameras to match. I learned something
]