How Much Lemon Extract do i use instead of real lemon or lemon juice?

Don't substitute extract for juice. It's not the same thing. Extract is more like the oil from the skin of the lemon and has no or very little water, which is mostly what lemon juice has in it. Lemon juice also adds some acidity for tartness, as well as having an effect in other ways but it doesn't have a lot of lemon flavour in baked goods. Can't tell you what effect the acid has exactly, I'm no chemist. If I really had no choice, I might substitute water and vinegar, half and half, for the lemon juice. That would give the needed acidity, and the vinegar smell will dissipate when the loaf is baked, trust me here. Use the extract as you would normally use an extract for flavouring, a teaspoon or two in a recipe. Ah, here we are. I just checked my reliable old Joy of Cooking, and it suggests substituting 1/2 tsp of vinegar for 1 tsp of lemon juice.

It would depend on how much juice the recipe calls for. You can sub extract for juice. Edit with the info and we can help more.