This question requires understanding the numbering of U.S. presidential terms.
Step 1: Identify the presidential numbers of recent presidents.
Barack Obama was the 44\textsuperscript{th} President.
Donald Trump was elected after him and served one term, making him the 45\textsuperscript{th} President.
Joe Biden was elected after him, making him the 46\textsuperscript{th} President.
Step 2: Determine the number for a potential future term.
The question asks what number president Donald Trump would be if he were re-elected. The presidential number is sequential and assigned to the person, not the term. Since Joe Biden is the 46th president, the next president elected after him will be the 47\textsuperscript{th} President, regardless of who it is. If Donald Trump were to be re-elected, he would serve a second, non-consecutive term as the 47th President.
Note: Grover Cleveland was both the 22nd and 24th President, serving two non-consecutive terms. Because of this historical precedent, a person re-elected after a gap receives a new number.