depends what your looking at achieving.

If your looking for a decent sound just for your own pleasure then 2 components and 1 10" sub behind a seat will be more than enough! reason you dont need rear speakers is because the mr2 cockpit is very small and the rear speaker are VERY close to your head.

The sound from the front speakers will reflect from the back of the car easily

If its to show off at cruises then i do not know whats recommended or if there is even a limit!

Im going for Hertz energy components 5.25"
Hertz 4 channel ep4 amp

(will be bridging 2 of the channels to power the sub)
And 10" sub behind the drivers seat, I dont know what sub yet but i will post on here once i know.

Music i listen to mainly is Rock, Drum

& Bass and many sorts of Dubstep