1st we create base layout for sculpting then we sclupt whatever we want, then we retopo it, but now i get confused because after making retopo of it we again apply multiresolation modifier in retopo model and sclupt our final detail and then we agan make retopo and bake our final details on it .
1. why we sclupt it again with multiresolution modifier .?
2. should we need again retopo it on , [after multires. model ] final detailed sculpted model.?
you shouldn't need to retopo a second time. The multires modifier is non-destructive (as long as you don't hit apply, it preserves your retopo'ed mesh), but you can bake the sculpted detail into a texture map (normal or displacement depending on what you need) and then use that detail in the material creation process on your low poly retopo to get some free details without extra geometry.
So...
1. you do an initial sculpt to get the form you want without having to worry about edge flow
2. you retopo to get the edge flow you want without having to worry about form
3. you do a sculpt pass with multi res to get fine detail for texture baking
4. you use the low poly retopo asset for production
hope that helps. it is a bit of a dizzying process.