The original description uses the value 12 because of the prior value of 2 hours, whereas you seem to just have N generals and no such hourly time from which to derive 12, plus this:
The existence of the 12-block chain is proof that a majority of them has participated in its creation.
Why is the existence of 12 verifications proof that a majority of N have participated?
because an attacker controlling less than 50% of the computational resources can be lucky and find the first solution before anyone else.
he can even be lucky twice or thrice. but to be lucky 12 times against everyone else with less than 50% of the resources, is statistically improbable.
2 hours is the result of those two values (12 being statistically relevant, and 10 minutes a chosen parameter of problem difficulty), not an input variable.
The existence of the 12-block chain is proof that a majority of them has participated in its creation.
Why is the existence of 12 verifications proof that a majority of N have participated?