Approx 60-70% is ideal
Avoid exceeding 75% (unless during shed)
When you think of a ball python's natural environment, you probably think of dry, arid habitats. This is the case, however, ball pythons find and reside in places with higher humidity. The most frequent locations are termite mounds or underground.
The amount of air moisture in your ball python’s enclosure is very important, yet still manageable. People have had the most success when keeping their ball pythons at approximately 60% to 70% humidity. If your humidity is too high, you can expect to end up with respiratory issues, scale rot, and other gross stuff. If it’s too low, dehydration and trouble shedding are two potential outcomes.
Ball pythons are very hardy animals and I'm often impressed to see them still alive in some of the worst neglect and conditions. Of course, avoid this as often as possible, but if you don't get humidity perfect right off the bat, don't panic! Just play around with your husbandry until it stays consistent.
Simply adding a nice substrate choice and large water bowl, in addition to the heating already in the enclosure generally creates enough evaporation and moisture in the air, bringing the humidity up to the correct level.
Measuring your humidity is easiest with a hydrometer.