Number 9's are the standard for the 750 ml bottles, as well as the 1 L and 1.5 L Magnum sizes.
Number 8 size corks are for the smaller 375 ml bottles.
Be careful when using the two metal finger style cork removers, rather than screw type pullers, on a #8 cork in a standard size bottle.
The cork can push IN...dousing you, and others nearby, with wine...almost always RED, too!
I've even found some commercially bottled wines, especially Gallo, using the smaller sized corks in standard bottles.