This is more complicated than you would dream because when bound you do not want sequential pages in the printing. That is, on your first sheet you might want pages 2 and 47 on one side and 48 and 1 on the other side so that when it is bound with a staple or ring binder on half-size pages it will line up.

I assume that your merge is to a separate document rather than the printer (or pdf).

See Booklet printing: Or how to print a folded booklet with numbered pages half the size of your paper