From Project: Redcap
Multiple Casting is an ArM5 mastered spell special ability that lets a magus cast the spell several times at once. This allows the caster to affect multiple targets in a single combat round.
The wording of the rules is a little unclear about exactly how many versions of the spell a magus may cast. Consider the following to be errata:
Multiple Casting (p. 87): Replace the the last sentence of the first paragraph with "You may cast a number of additional copies of the spell equal to or less than your Mastery Score."
--David Chart, Berkeley Ars Magica List, 17 Oct. 2005
So if your mastery score is 2 and you have the Multiple Casting special ability, you could cast up to 3 instances of the spell simultaneously: the original plus two additional copies.
The reason this is a Frequently-Asked Question is that the above errata were not corrected until several months after ArM5 was published.
- ArM5, page 87