When Blight is added to a land (such as during the Ravage step), one Presence from each Spirit with Presence in that land is destroyed, regardless of how many Blight are added or how many Presence are in the land. If a Spirit has multiple Presence in the land, only one of their Presence is destroyed.

This is specified on page 15:

After Setup, whenever you add Blight to a land (including when Blight cascades), two bad things happen: • Destroy Presence: In that land, destroy one Presence from each Spirit with Presence there. Destroyed Presence is placed next to the Island, not returned to the Spirit Panels.

There is no mention of choosing which specific Presence is destroyed if a Spirit has more than one in the land, so the choice of which Presence to remove (if a Spirit has more than one) is up to the player controlling that Spirit.

If there are more Presence in the land than Blight added, it does not matter: the rule is always "one Presence from each Spirit with Presence there" per Blight addition, not one Presence per Blight.

Source: page 15.

Source: Core Rulebook (pp. 15, 21, 8, 9)