Does a 26 of alcohol refer to a 700 or 750 ml bottle? Ive heard both

It's a 750 ml. It refers to the size in ounces (more accurately, to the size of similar bottles before metric was adopted; a 750 is actually around 25.5 oz).

It refers to 26 fluid ounces which is ~770ml, so the closest size in round metric numbers 750ml. A 700ml bottle would only be about 24oz.

Looks like 26 oz is about 769ml, so maybe the 750 is a "close enough" equivalent?

750 mL is about 25½ fl. oz.