Question on number of plants

I hear a lot of people saying that there are 16 different crops but if i add them all up ( not including weeds ) I get 18. Can someone tell me how many possible different crops can be grown using a crop? This includes the none crossbred crops such as wheat, melons, pumpkins, reed, sugarcane, roses, and yellow flowers.

Thanks for the help. Now I know how many more crops I need to discover.