The estus flask upgrades to +7 likely because there are only seven unique fire keeper souls. One in Undead Parish. One in New Londo Ruins. One in Blighttown. One in Duke's Archives. And then one each for the three surviving firekeepers (Firelink, Anor Londo, and Quelaag's sister). It is possible to fully upgrade the estus flask without killing any firekeepers with just two game cycles, in fact you can go from +4 to +7 at the very beginning of NG+ by using the master key reverse shortcut path. You will need to kill Quelaag to get access to her sister though as collecting both the Blight Town and the Undead Parish firekeeper souls will trigger Lautrec to kill Anadtacia before the bells are rung.