I run one steady and one blinking. The blinking grabs attention, and the steady gives a distance and velocity focus point. And as someone else mentioned, having two means if batteries run dead on one, you still have one other.

More than that might be better, but you are probably looking at diminishing returns at some point. Perhaps an additional one high up, ie. on the helmet, to be seen by cars farther back in a queue, etc.

It depends on your commute route too - traffic volume, speed, etc. I commute on 30 mph city streets, and honestly, with the two rear lights I am running, I've never had a conflict from the rear. All of the weird traffic behavior on my route tends to be turning vehicles and intersections. YMMV.