simran,
I think it's best to take it with food because it's not intended to replace a balanced diet. How much depends on the individual. It depends on what the rest of your diet consists of. If someone consumes a lot of omega 6 from meat, eggs etc., then they likely will need more omega 3 from fish oil or flaxseed oil. On the other hand, if a person gets a lot of omega 3 from eating fish and leafy greens, they might not need a supplement at all.
It's all about getting the proper balance between omega 6 and omega 3. In my opinion, the best way is by eating a balanced diet that has enough omega 3 (and not too much omega 6).