You can use a retriggerable one-shot, whose period is somewhat longer than that of the pulses. Thus, each pulse will re-trigger the one-shot. After the last pulse, however, the one-shot will not be triggered and once it times out its output pulse will end. So that is one pulse for no matter how many input pulses.
Check out the 555 datasheet, I am almost certain there is an example of such circuit.
The only problem is if you really need the output pulse to end EXACTLY on the falling edge of the last pulse. The circuit I am describing will ends its output pulse after one period (its own period) after the last rising edge received (after the rising edge of the last pulse)