The relationship between external(leaf) nodes and internal nodes for binary tree is: $$L = i +1$$
According to ques, $i = n$ so using formula,
$$L = n+1$$
Since tree is made up of internal and leaf nodes
Therefore, Total no. of nodes(N) = $L +i$
So , $$N = n+1+n$$
$$Ans:N = 2n+1$$