Keep indoor humidity below 60%, ideally 30 to 50%
Added
Use a cheap humidity meter and run a dehumidifier or air conditioner when indoor relative humidity climbs. Also dry spills within 24 to 48 hours, clean and repair roof gutters, and vent dryers and bathroom fans outdoors so moisture does not feed mold.
The result
Result: EPA: keep indoor humidity below 60% relative humidity, ideally between 30% and 50%; drying wet materials within 24-48 hours usually prevents mold growth.
Source
EPA: A Brief Guide to Mold, Moisture and Your Home (opens the original source in a new tab). Check it for the latest numbers before you act.