What is the inflation rate of Stratis?
EDIT: Oh I just saw: Just 1 Stratis per block with only 12500 blocks to be mined. After that PoS.
what will happend after POS is started?
No mining, just staking.

Is there are benefits from staking?
yes, you get rewards of 1 stratis each POS block you stake.. also supporting the network security.
suppose i have 10 000 stratis. What amount of reward i will get and after what time?
no one really know. you have to stake to see the result yourself but i doubt you will get much. it depends on network and your weight. your coins will be staking after 50 confirmations of course with small amount of coins you probably wont get rewards right away