Why can't I choose even numbers? Because in traditional culture, the even number is "Yin", which means unlucky and unhappy, while the singular number is "Yang", which means positive. In geomantic omen, the singular number is Yang and the even number is Yin. If the number of words in couplets is even, it will easily lead to Yin Qi.
Therefore, it is necessary to post odd-numbered couplets, and the odd number symbolizes auspiciousness.