How many coins are needed to stake? If I get may wallet on a PI and leave it open I will get 20% yearly with any amount of coins?
Yeah. But I would recommend you to start with 1000 coins in a single transaction, you'll stake faster.
Proof Of Stake is very energy friendly in the way you don't need anything else than your CPU which won't be used at full speed either.
Mine elsewhere, convert to BTC, go to an Exchange, buy your XST.