I don't think you can do this with onPlayertouch, but maybe with zone events
You could for example make something like this
onactivate -> self ->modvariable [peopleinzone] -> set
onplayerenterzone -> self ->modvariable [peopleinzone] -> add[1]
onplayerexitzone -> self ->modvariable [peopleinzone] -> subtract[1]
and then a check that triggers whatever you want:
(1 second delay) onplayerenterzone ->ifvariable [peopleinzone] >= [5]
onvariabletrue -> blablabla
I haven't tested this, though :/