@atulcse For an unambiguous grammar every string produced by it will have 1 parse tree. But for a given string many grammars may produce it and some of them can be ambiguous too. If that grammar is ambiguous so there will exist more than 1 parse tree for that given string according to that ambiguous grammar production rules.
A grammar is ambiguous if either 2 or more parse trees/ 2 or more LMD / 2 or more RMD can be made for that grammar. So @adad20 you are right.
For a given string and some ambiguous grammar producing it, is #LMD = #RMD always true?
I don’t know about any such relations :(
May refer this, though not sure : https://gateoverflow.in/168440/Derivations