The BO have killing and APTX and can try to manipulate the lynching, which will reveal them.
Some towns have APTX too if they get it. They can lynch, protect/heal at night and arrest! So they actually have enough methods to take the BO down. While the BO could reveal themselves easily whit night killing or lynching or APTXing.
The problem is, that the town doesn't know anything at the beginning. So, possible miss-lynchings are almost a given. But at the end, the town usually knows a lot of BO's already, or more the other way round, they know who is town. So the ones not revealing anything are usually suspected. And if you give even less BO's, the BO doesn't have a chance to not get lynched at the end, and it's almost game over for the BO.
I think it should be up to the GM how many BO's there will be. And shouldn't be a fixed formula.
But more like guidelines.
So in case there is Tequila in the game, the BO shouldn't be more than (players*1/3) -1
If there are a lot of police in the game, the BO should be rather (players*1/3).
As long as the GM thinks it's balanced enough. Since the GM doesn't know if the BO is lucky and kills a police on night 1. Or if the town is lucky and get's some arresting or successful protection done. After that it depends on how the players play and how lucky some people are (since tommy interrogating beastly was luck for us BO's

)
There is also the chance that some people won't be able to log in because they are busy or their inet died. So like half of the BO was MIA at some point in round 11. Of course that can happen to the town too, but it's more crucial for the BO to have teamwork or using their abilities at all. (or getting the Black suitcase ;p)
So maybe giving the BO more rights. Like the last in the ranking can give a Black suitcase order too (and of the other players action, like slandering etc.). And the next one in line can overwrite that and the one below can't change that anymore. SO if the leader does a order, it can't be overwritten at all.
But in case the actual player with the ability (gin for slandering) gives an order for his slandering, then it can't be changed by anyone. Just in case he didn't give an order at all because he's busy etc.
Voting is the same issue D: