Normally, it would be 10, but you are always free not to use your magic, so it's in fact 12
I'll look up a page reference this evening.
Edit: The rules are in Street Grimoire p.32
A background count impose a negative dice pool penalty
equal to its rating for all tests linked in any way to magic (such
as spellcasting, summoning, and skill tests that use active adept
powers such as Killing Hands or Improved Sense). The exception
to this rule are background counts from domains, where a tradition,
person, skill group, or skill may be exempt from the penalty as they
are used to or aligned with the domain. Dual-natured creatures
or purely astral creatures take a negative dice pool penalty to all
actions equal to the background count. Again the exception of a
domain can apply to specific types of creatures or spirit types.
Pre-existing active foci, sustained spells, quickened/anchored
spells and rituals are reduced by the background count. If they are
reduced to 0 or less, spells fizzle, wards and rituals collapse, foci
deactivate. A foci cannot activate while under the influence of the
background count. Anchored rituals and quickened spells if they
have not expired revive themselves at 1 point of Force per hour,
up to their preexisting Force. Preparations triggered while within
a background count have their potency immediately reduced by
the background count. Adepts may use a Simple Action to turn on
or off a passive power in cases where penalties from background
counts might exceed bonuses from their powers.
