She likes to root for the underdogs
She likes to root for the underdog
I'm confused to decide which form to use here, plural or singular !
Here, underdog is used as a general term for a player or team that is not expected to win. It is not specific as is.
So I think one would generally say:
She likes to root for the underdog.
She likes to root for (the) underdogs.
this may not be technically wrong, but it sounds odd and I don't think it is what anyone would say.