Thanks guys for your answers. I'm sorry , maybe I did not ask exactly what I wanted. So, what I was trying ask is if somebody has a calculation of how many interrupts can a Mega 2560 read per second with an optimized ISR.
@AWOL, I know that less than 16 Million per second, but let´s say that the first limitation is the clock speed.
@MarkT, @DrAzzy OK.
@Jurs, sounds good, this could be a good estimation, 200000 interrupts per second ( I can read from an Instrument until 200 kHz pulses).
@UKHeliBob, just 500 Hz, but could be until 3KhZ.
My ISR just reads from a pin, and counts the time of each pulse (using micros() function), so I think it will be able to do it with no problems, right?