You need about 2 million yen in japan to survive in a frugal way. So assuming dlsite takes the usual 30% cut and taxes are another 20% a doujin game maker would have to make 4 million yen a year to survive which would be around 2666 copies they would have to sell in a year of a game that costs 1500 yen. So with hidden costs I'm too lazy to factor in it would probably be closer to 3000 copies.