how many times one can change his/her citizenship?

I don't think there is any fixed limit, but the citizenship process may take years if your application is not rejected. Why would you want to change citizenship more than once or twice in your life? If you do a lot of traveling between two countries, you might consider applying for dual citizenship.

As many times as another country will let you take on citizenship, but I'm guessing that after you have changed citizenship once, most countries would be very wary of your intentions. Remember, the US has one of the most lax citizenship requirements and it takes years to become a citizen.

Those countries may not want to take you!