Approx 30-50% is ideal
Don't exceed 60% (other than during shed)
The amount of moisture in your corn snake’s air is very important, yet quite easy to maintain in most cases. 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 potential outcomes.
Thanks for us corn snake keepers, extremely specific levels aren't super important, and you will generally only run into issues if you're in a very humid climate. The general consensus is that your corn snake's enclosure should not maintain levels at or above approximately 60%. Doing so has resulted in the issues mentioned above.
Ideally, your enclosure isn't as dry as the Sahara desert, but keeping it somewhere in the 30 to 50% range is what I've been most successful with. Adding a bit of moisture while your corn snake is going into shed isn't a bad idea, as it assists your snake by loosening up the old skin and increasing the chances of your snake shedding in one piece. (This isn't always necessary though).
Measuring your humidity is easiest with a hydrometer.