aseem123 :
How much should be the clearance between PSU and motherboard please help me?
More than a negative number
Generally the issue is that there are obstructions that limit the PSU length (grooves, panels, screws, etc), but if the only thing between the PSU and the motherboard is air, generally you'll want no less than 2 inches between the back and the motherboard. Everything depends on where the PSU is (again, that garbage of a maker for your case doesn't even say where the PSU is mounted!), how flexible the PSU cables are, and how much overlap between the cables and motherboard you are comfortable with.
If you haven't purchased the case yet or have enough left over for a new case, please just scrap that thing for one from a reputable company (literally a dozen better companies out there), and then it becomes very easy because basically every reputable company gives you full schematics.