As an Indian who's lived almost 4 decades in India, I have never heard of "ek number" as the response to a joke, or even ever used in the stated context of the joke being rated no 1. The closest I can think of is the phrase being directed at the joker, not the joke; a sort of "Oh, you're a first rate a-hole for cracking that joke". It's a terminated "Ek number ka ... (haraami?)", which is the only form I've ever heard this phrase.