There are several FTers with three or more citizenships. I can think of one with four. I have also come across the odd individual with one American and one British parent who was born in Canada (three times actually this very combination).

Someone once claimed to have six but disappeared and never answered my questions about it.

I personally have two citizenships (US and UK) and am equally allegiant to each. Thankfully neither country much cares. It is very common in this country to meet people with multiple citizenships, normally children of Britons (or other Europeans) who have emigrated, or immigrants who have been naturalised.