How many levels of Spell Mastery do you need before you can teach a willing student?
It's an Ability as any other.
From p. 86: "For every possible Hermetic spell, there is a corresponding Ability. This Ability can be studied in the normal ways, and is called the spell’s “mastery” ability."
From p. 164: "A character must have a score of at least two in an Ability, or at least five in an Art, before she can teach anyone."
I thought this was probably the case, but I also thought there was an errata explaining some differences between Spell Mastery and other Abilities that might have modified it.
From a metagame POV, I can say all the Spell Masteries in the core book are derived from Flambeau the Founder's signature spell.
Other Founders may have known other Masteries, but none of them had Spell Mastery at level 2 or higher, so as to teach Bonisagus.
The errata were issued because otherwise each Spell Mastery should have a specialty, as was done in one of the original books but was not intended. That was the only thing being fixed.