I'm not sure what you mean by fiance. If you mean fiance visa, that's a nonimmigrant visa and one would use I-134, not I-864; it's only when they're here and married and apply for Adjustment of Status that they need an I-864. If you mean you will marry them and immigrate as a spouse, then that's different.
On each I-864, you would have you, your spouse (once you're married), and the immigrant (if not the spouse), for a total household size of 2 or 3.