I have heard and also read in various threads that mining is too risky.
You heard it right and mining (legit) is risky today if you don't have your rigs to mine for and you just suddenly think that it's a profitable one. But if you are referring to cloud mining, 100% it's risky and not recommended.
The invested money can all be gone with a blink of eye. Actually i am not sure wither i should do it or not. If i invest, then is there any chance to loss all the money?
You had already said the answer on your question
If i invest, then what should be the amount first?
Go for trading instead and not for cloud mining.
And in which sites would be better to do mining?
Cloud miners are trusting genesis mining but just like hash ocean it can runaway anytime.