I like humour, as everybody does. Recently I met two fellows who started talking about the election scenario, and talking themselves, they cracked so many jokes, most of which, they invented extempore, on the spot, I mean. They did not leave any hole or loophole in what ever they talked about going untouched by their great sense of humour.

But, I feel often short of words, or humour, in such a discussion. How can I acquire great sense of humour?

I'm not sure you can.
I had a dad with a constant, quiet sense of humor and a mother with none.
I have aspired to have one all my life, but it just isn't in the cards.
I like haha jokers, I like dry wit, I like thoughtful humor, I like puns, but I also like plenty of people who are very serious. Even the most serious people will say something once in a while that is spontaneously funny; who needs it every minute of the day?
I've known nerdy types who had pun contests (you can go to pun competitions) for as long as each person could last, and they were very good at it, but I wouldn't say they were especially funny people in general.
Just be who you are, likable.