For me it's not the number of joints she's smoking that's the main issue.
If she's smoking in front of the kids, that's the issue to me. If she wants to do pot then that's on her, but the kids don't have a choice. The least she can do is take it outside. There's no way she's doing that if she's smoking 30 joints a day and caring for all of these kids.
Thirty is excessive, and I don't see how she can be a good mom when she's stoned 24/7, which she must be if she's smoking that much.
I don't have a minimum number to offer.
If she smokes even one joint in front of her kids, that's when I'd rat her out.
If she has a few a day, but doesn't do it in front of her kids, continues to care for her children properly, then I don't have a problem.