Honestly, grouping plants together helps tremendously with the humidity. There are a couple of them that will always need more (for example, it seems no matter what I do I can't keep my alocasia stingray or philodendron aurea thriving) but truthfully, I should be keeping those in a different environment anyway. Most of them are absolutely fine. During the winter it does get really dry in there (I'm in zone 4, so heavy and long winters) so I run a humidifier if it gets below 45 on the hygrometer, but it usually doesn't because they're all so grouped up.
I'm more surprised that my cacti seem fine with the humidity levels and never get signs of rot. I dont know if it helps but I keep all my dry/arid plants on one shelf, and my more humidty-needing ones grouped together away from the arid ones.
I have a thermo/hygrometer but it always seems to stay stable.