smc

From the first column, we see $A+B<10$ because it yields a single digit answer. From the fourth column, we see that $C+D$ equals $D$ and therefore $C=0$. We know that $A+B=D$. Therefore, the number of values $D$ can take is equal to the number of possible sums less than $10$ that can be formed by adding two distinct natural numbers. Letting $A=1$, and letting $B=2,3,4,5,6,7,8$, we have $$D=3,4,5,6,7,8,9⟹\boxed{7}$$