1- How many presidents have preceded Obama? Natural. The answer is 43.
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Anonymous12- What number president is Obama?This is the only attested form I know of from personal experience, believe it or not.
AnonymousIt's informal, isn't it?Yes. I suppose the formal version is something like one of yours:
Anonymous12- What number president is Obama?In everyday English I think this is a good solution to this perennial question.