How many coins needed for staking?
The more coins you have - the more staking rewards you will get! If you are holding small amounts of coins, you don't get a lot of rewards.
That means, that for staking I just need to hold some amount of coins? Should I have to unlock anything?
Yes, you can stake even 1 coin. But the fewer coins you get, the less the chance that you will get your staking reward. Staking goes by default, but please make sure to unlock your wallet if it's encrypted.