Buddy, I didn't see this question before, or else I would have answered.
You know I love Mage... at least Ascension. I say she can affect more
than one bullet, provided she has enough successes.

Going by the rules on pages 208 and 209 of the last edition of M: TA,
and a little extrapolation of my own based on the description of the
Prime 2 effect "enchant weapon" (page 183), she would have to split her
successes between number of bullets, and duration of the effect.

She will need one base success for the whole effect, then, say, one
success per two or three bullets (your call depending on the caliber and
type, I'd say you can affect more .22 slugs than you can affect .50
ones, or more .45 than .45 explosive rounds). This I just extrapolated
from the area rules of page 209.

Then there's the duration of the effect, straight from page 209, with
the base success spent for success it lasts for one instant or turn; one
extra success spent on duration makes it last for a scene, two make it
last a day, three a full story, four a month, and five it's
storyteller's option. Scoring double the normal successes required for a
complete success on the Effect (the whole thing, not just duration)
might make it permanent, at Storyteller's option.

I any case, as per page 183, this requires a point of Quintessence to be
spent, possibly more to make it permanent.

MaGnUs

PS: Do I know this player?