250 is simple answer but it depends on the amp and the ohms rating for the speakers. IF you have a stereo ( 2 channel) amp with 1 speaker on each amp channel and the speakers each have the same ohms rating (say 8) then your available power is evenly split. But the actual power you are putting out at any given moment depends on the music applied (frequency) and the volume setting on your amp. Forget the 500 watt
info as that's almost useless taken out of context. normally correct power specification should read like "x" power from 20Hz to 20Khz into an 8 ohm load. Also, power ratings on speakers can be misleading and/or misunderstood. better to match the ohms rating to what your amp is looking for.