After I posted this question, I found that it had been discussed in many forums and there are no definite answers. It seems there are different ways to express this idea in different regions and even among different people.

Phil, the Mandarin equivalent is the same, i.e. 第幾任(traditional) or 第几任(simplified).

係Google淨係打"What number of president was" 就出現一堆名字

What number of president is Trump?

What number of president was obama?

我唔係講呢個係正確答案，因為很可能呢個問題冇一個正確答案!

Hello. There is no such expression in English. 

But you can translate that into the following sentences: 

1. What number president was Barack Obama? 

2. Which term of U.S President did Barack Obama serve as?

:)