Please fix your profile, there is no character with that name on that realm.
As Unholy, at what point should one replace Scourge Strike with Blood Boil, i.e. how many adds are required?
A simple comparison of damage in simcraft seems to show that Blood Boil does more than half as much damage as Scourge Strike, on top of which it spreads diseases and the entirety of its damage can crit, so it seems pretty clear that you should Blood Boil if there is even 1 additional mob present.
What if you are performing a Festerblight priority? Now BB should definitely be used if procs are present, but what if diseases would be spread significantly weaker than what is already ticking on adds?