Do you think there's more than seven Half-Bloods in the Next Great Prophecy?

Unarguable and best answer: We'll find out before the series ends.

He seven half-bloods might be the ones who fight the Giants, but maybe a few more supporting characters will help them along the way.
Like, Annabeth isn't directly mentioned in the first one, but she did help out Percy a lot.

No. It would be too hectic and crazy to write. You couldn't really focus as well as you could with less characters.
So read this Ricky!

There will probalay be more demigods to help. Like Annabeth she isn't one of the seven but she's probalay going to help out. Or Nico and Thalia I really doubt that they would be one of the seven even if their powerful but RR wouldn't make it like that but they're both going to help out alot. Like Maybe Thalia might help Jason and their father fight his giant counterpart, and Nico will help his dad in the underworld with the the doors of death or something