The given conditions define the function values for overlapping regions of the input space.
By constructing the Karnaugh map using the given constraints and simplifying the resulting Boolean expression, the minimal sum-of-products form is obtained.
Counting the literals (variables or their complements) appearing in all product terms of the minimal SOP expression gives:
\[
6
\]
Final Answer:
\[
\boxed{6}
\]