I used around 6 to 10 dimetrodons in a house for incubation.

Doesn’t matter how many you use, just make sure that the insulation in the room is 2100 or better.

To check the insulation in the room, just take off all your clothes and open up your inventory screen…. It’s in the center console where it displays time, day, amount of tames and temperature… It probably says insulation count under temperature.