Half the park would be 9 coasters down. I don't think it's come to that, even in the off season.
Also, can't really blame the park for closing (especially on a weekday) with rain predicated throughout the day. Unlike Disney, the rides at SFMM (and Knott's, for that matter) being largely outdoors don't operate so well in the rain, so there's not much point in remaining open.
SFMM can be blamed for many poor and questionable decisions, but closing on account of rain isn't one of them. And SoCal has been experiencing more rain than it has in decades.