Counting pulses using Arduino Uno
As Peter Bennett and other pointed out, initially you had set 3Vpp, or a pulse alternating +1.5V to -1.5v.
As per the Electrical characteristics of Arduino's Controller, 1.5V is not guaranteed to be read as '1'. It should be atleast 0.6 of Vcc. Hence your irq pin cannot recognise edge transitions, instead it reads a '0' all time. And, you are not supposed to drive -ve voltage to the I/O pins as well to avoid damage !!
The pulse frequency is 20 Hz or 20 pulses per second. With 500ms delay, you are counting how many pulses occur per half a second, so result should be of value 10. That's what you are getting displayed on serial monitor.