I have talked to many BB employees, and they say that you have to do about 10 returns in a week before you're flagged. I do around 3 returns per week, and I think that's a bit high, but it's totally fine.
Many CAGs should be safe.
There is more to it then what you say as there have been plenty of people with less returns in a week then get banned. Working with the forum mods and customer cases of bans it seems to be a weighted system with added flags.
This is all speculation but what I've seen:
It weights money in returns vs money used for purchases the most. Ie you spend a lot then you can do more returns.
Secondly it weighs large ticket items differently then small ticket. Mainly because of the above. Sometimes for the customer sometimes against.
There's flags for quantity returned vs quantity bought. Ie you return 50% of your items regardless of prices etc.
There's time frames also. As no return for x number of days seems to lesson the strikes against you. Or possibly what you mentioned the x times in a week.
Probably much more also. Just small things I believe are part of the equation.