Lion Airs website lists 103 B737s and Batiks over 40 A320/B737. With the population of Indonesia and its rapidly growing middle class spread out over 18 500 islands a fleet size of around 400 aircraft is certainly possible. There are already off shoots of Lion Air operating in Thailand and Malaysia.
Obviously a serious player with considerable resources behind them. Their safety record is highly questionable but outside help has been brought in to try and improve things.
CASA have shown that they will not hesitate to ground anyone not playing by the rules and this would be top of the list of things to get right.