if we have to add two n-bit numbers by using half and full adders
To add LSBs we need a Half adder as only 2 bits are to be added as there is no carry
For rest of bits full adder is needed as carry from previous bits is added to next two bits
so we need $n-1$ Full adders & $1 $Half Adder
here n=16
15 Full adders
1 half adder