Too much humidity isn't the concern. Too LOW of temperatures WITH high humidity is the big concern.
If you have high humidity (60-80%) you must keep your temperatures above 80F day and night, in all areas of the enclosure. Lower temperatures can cause respiratory problems and a sick tort.
If there is an upper limit, I have not found it. I find that keeping it 80% or higher helps grow them smoother. Even in my 80% humidity closed chambers, they still have a humid hide that is at or near 100%, and they choose to sleep in it every night.