According to Libris Mortis (p. 80):
...the grafted creature can use the ghostly arm to deliver an incorporeal touch attack that deals 1d6 points of damage, the equivalent of a ghost’s corrupting touch attack. Treat the arm as a secondary weapon, but because it is incorporeal the arm has no Strength score and therefore gains no bonus or penalty on damage rolls from the user’s Strength.
If I already have 2 weapons and attack with them, using 2 weapon fighting will this give me a third (natural) attack? If i have 2 ghostly arms will it give me 4 attacks, and so on?