During the day in the winter time my tanks with bettas in them stay around 70F-72F and at night I wrap them up with towels to keep the heat in. But not sure what they stay at in the summer since I didn't have a thermometer back then.
the room is heated but bizzarely while one betta does fine the other looked unhappy so ive floated his tank in my bigger tank for now , might invest in two heaters though if i see a sale
Of course if you wanted to be really accurate bearing you could heat a whole shelf of bowls/tanks which may be about 5 or 6 you could attatch it up to a thermostat which would set you back a bit more but placing the probe in one tank and you could keep them all pretty exact.