For those wondering, you can use this item infinite times. It will allow you to summon a third phantom in most areas, but also allows you to be invaded by another. You can summon up to 5 dark phantoms if you use it. It loosens SL restrictions, making it so you can be invaded by someone who is around 70 levels higher than you. In Anor Londo and the Crucifixion Woods, you can only summon two phantoms, although the 5 dark spirits can still be summoned just fine.