Kamala would be be President for the remainder of Biden's term. That would make her the 47th President. Therefore when Trump took office a few days later, he would be the 48th. All his merch that was produced would be incorrect and waste his and his supporter's money.
As far as I know it wouldn't, but Kamala is VP, so if he resigned she would be the 47th president for the remainder of Biden's term. The joke is that the trump merch says 47 but now it would technically be 48 and they'd have to make new stuff.