how many coins i need to hold in 1 block in order to get a reward?
As I understand it, there's no lower limit. But you'll wait an awful long time for a reward if you have 1 coin!
how often will i get the reward?
Depends how many coins you have, like most PoS. The more you have, the more likely to stake.
will it be 1000% apr forever?
It's 100% for another week or so, then 36%. You didn't read the fork info.
also price is now around 300 satoshi, what will happen when we will hit inflation and get closer to 25 billion circulate coins?
Price is 1 sat on all exchanges, unless you're in the DOGE market. It sometimes jumps to 2,3 or 4, but there's a big sell wall to get through first.