It's up to you. What I did was release the 6 girls that I had in the beginning, well 7 actually sorry. 6 were all sisters and 1 was from PetCo. I actually didn't QT them since I know they had spent 2 weeks at my LFS in their own tank and they're super good about keeping fish well. So I went on a whim and trusted that they were just fine and it turns out they were
So anyway, 7 added all at once and from there when I get new girls I QT them in their cups floating in the big tank for at least 2 weeks. I usually let one out at a time, so I had gotten 3 more sisters and I knew one was the runt and getting beat up a lot so after the 2 weeks, she was first to go in, then her sister came out 4 days later and then the most rowdy of the three went in 5 days after that even though I got them all at the same time. However this was more for behavioral issues, not just because I didn't want them in at the same time. I've done it though where I've released 2 at a time and they were just fine. So it comes down to what you feel comfortable with and what the attitudes of the girls seem to be.
Although that rowdy sister, she's actually super calm once she's out of her cup. The thing that set me off was she accidentally got out of her cup at first when all three were in QT and she was flaring and ramming herself into her younger sisters tank. So they were literally going to kill each other. I cupped the rowdy girl back up and that's why I waited the extra week to let her out. The younger siblings had already established their places in the tank and the rowdy one settled down immediately. I haven't seen her peck another girl since!