As above, 1 or 2 will be fine.
My one got HUGE in 350L, i swear it was bigger than any website says they get... it was also the most boring fish i have ever owned! I literally had to use a long handled algae scrubber to poke it once a week to check it was alive still as it didnt move or eat from one month to the next otherwise (was overweight and not hungry). If it did ever move... it shuffled about 1ft and back again in the middle of the night...
but 1 in a 4x2x2 340L tank was more than enough as an adult. personally.... i wouldnt have had more... even if they are lazy! and no point buying fish that will likely overcrowd your tank in the future...