IMO you'd be good with 1 male and 3 or 4 females Scarlet badis in a 40 litre (10 US gal) tank and believe they like densely planted tanks with lots of hiding places, caves and rocks. Sand or fine gravel substrate and dim lighting is recommended.
sorry i was supposed to say i have 4 in a small tank.so i have got them.anyway i am changing my mind.They where in a small 25 liter tank but i have decided to put them in my 100 litres aquarium which will have tetras and cory. When i brought them the guy from the shop just gave me 4 of the biggest in the tank so i dont have a clue if they are male or female