Let the tree T is 3 ary tree , means N = 3
Root None
/ | \ m = 0 => 3^0=1 node
1 2 3 m = 1 => 3^1=3 node
/ | \ / | \ / | \ m = 2 => 3^2=9 node
at depth m , maximum number of nodes 3^m , we can see from this picture.
so if m is the maximum depth of any node of tree T then maximum number of leaves N^m for a N ary tree.