Support can target a creature another player controls.

If a spell with support has other abilities that target creatures, those abilities and the support ability can target the same creature.

You can't put more than one +1/+1 counter on any one target using the support action.

If some, but not all, targets for a spell become illegal, the remaining targets are affected as appropriate. If all of a spell's targets become illegal, that spell doesn't resolve.

You choose the target creatures to support when Jubilant Mascot's triggered ability is put onto the stack, but you don't choose whether to pay until the ability resolves.