The proper humidity for a cigar humidor is about 70% but a range of 65-75% is acceptable.
Too much humidity will cause cigars to swell and possibly mold. Too little or low humidity will cause cigars to dry out and possibly crack. Dry cigars will not taste good and if the wrapper is cracked, the cigar will be unsmokeable.
Use humidor packs to maintain the correct humidity. It’s the easiest way. When the pack is dried out just replace it.