Marijuana needs humidity of at least 40% to grow. During germination/cloning, a humidity range of 80 to 90% is required. For germination of seeds I use a clear plastic dome, this helps keep humidity up. When pot is in the vegetative stage I try to keep humidity at around 60%. During flowering, it should be lower (50% range).
It’s not always easy keeping humidity at the proper level. From my personal experience as long as humidity is over 40% things should be ok. However try not to have humidity get too high (over 70%) during vegging and flowering, there is a risk of mold with high humidity levels.