four agtf can be kept in that size of tank. personally i wouldn't restrict ventilation because tree frogs like good airflow. i use eco earth as substrate and mist once a day and this is enough for me to maintain humidity of 60-70 percent. a lot of reptile hydrometers i've found to be unreliable to measure humidity, they'll work well for a week or so and then start to go bad. i use the zoo med hygrometer now though and seems to be good enough