Originally when we have root , there is only 1 node, which is leaf. (There is no internal node.) From this base case "+1" part of formula comes.
When we n children to root, we make root internal. So then Total Leaves = = 1(n-1) + 1 = n.
In complete n ary tree every time you add n children to node, you add n-1 leaves & make that node to which you are inserting childen internal.( +n for leaves, -1 for node which you are attaching ). So if you had originally few leaves, you add n-1 "New" leaves to them. This is how x(n-1) +1 makes sense.