I have seen the super cute swirly numbers on top of cakes. It is just fondant? And what do you use to get it to stand up? Thanks!

Some people use straight fondant, fondant with some tylose/CMC added, 0/50 fondant gumpaste, or gumpaste. I find it best to insert a skewer (or more than one, if need be) into the bottom of the number while it's soft and let it dry on that, an then insert the skewer into the cake.