ordinals - How to frame a question to get answer about the turn that somebody has taken in doing something?

I want to frame a question for which the answer will be :

Obama is the 46th president

I am not looking for "who is the 46th president?" I trying to ask a question where I want to quiz the other person on Obama's turn (46) as a president.

For instance(incorrectly)

"Howmanyth" president is Obama ?