Hi depends on the B/Ns. I currently have 3 baby b/Ns in a 13 gallon,2 adults in a 40 gallon* and 20+ in a 90 gallon,also had 60+ in a 40 gallon all with no problems.
* These two have bred and i can count at least 7/8 babies in there.
Now to a strange point, my original adult male B/N* is nearly 4 inches long so yes at this size a 20 gallon is not big enough as they are quite territorial, but all me other B/Ns are noticeably smaller,hence the higher stocking levels? Also my tanks have loads of cover and plenty of hidey holes.
* This male is at least five years old.
I would say if samt1 re-homed the plec then if samt1 got 2 possibly 3 baby B/Ns then they would grow up together with know problems in my opinion. Also would be preferable to have one male to two females if possible,but they are hard to sex when young untill the male develops the 'bristles'