Perhaps a picture would help.
See the picture at he bottom of this page under '2. Left Child - Right Sibling Representation'
In the diagram, each node has three fields: a value (at the top) and and two pointers (in the bottom two cells)
In the left cell at the bottom is the pointer to the first (left) child; and the right cell has the pointer to the next (right) sibling.
A non-null pointer is indicated by an arrow pointing away from the cell, and a null-pointer is marked as NULL.
There are eleven nodes in the tree: A, B, C, D, E, F, G, H, I, J, K n == 11
a. Total number of pointers == total number of cells at the bottom == n*2
b. Number of non-null pointers == n-1
(There is one pointer pointing to every node except for the root node.)
c. The total number of null pointers == total number of pointers - number of non-null pointers
== (n*2) - (n-1)
== (n+n) - (n-1)