I guess what is meant by the number of full parse trees here is the number of different parse trees that you can find for the whole sentence "the hungry lioness chased the careful oryx with patience" whose root is the starting symbol of the grammar i.e. S in this case.
Even though there are only 2 ways that the symbol S on the top cell is formed (S -> NP VP & S -> NP Y1), in "row 6, cell 4" there are 2 ways the symbol VP is formed (VP -> V Y2 & VP -> V NP). So, considering this if you construct the parse trees, there are altogether 3 different parse trees that we can formulate for the sentence "the hungry lioness chased the careful oryx with patience".
Even though there are only 2 ways that the symbol S on the top cell is formed (S -> NP VP & S -> NP Y1), in "row 6, cell 4" there are 2 ways the symbol VP is formed (VP -> V Y2 & VP -> V NP). So, considering this if you construct the parse trees, there are altogether 3 different parse trees that we can formulate for the sentence "the hungry lioness chased the careful oryx with patience".
