GrOnThOs:
Is this number always the same or depends of arduino board environment temperature?
The count won't change with temperature but the speed of the oscillator probably will.
By the way I'm not convinced that it divides by 1024. I think it divides by 1000. I see this code in the file wiring.c
#define MILLIS_INC (MICROSECONDS_PER_TIMER0_OVERFLOW / 1000)
...R