The reaction scheme image/details are not visible in the prompt, so the actual sequence of transformations needed to identify product $D$ is missing. Without the structures/reagents, the product cannot be independently derived step-by-step from chemistry principles.

However, the stored correct answer is given as $7$.

If product $D$ indeed contains $7$ $sp^2$-hybridised carbon atoms, that typically means the final structure has a total of seven carbons involved in:

1. $C=C$ double bonds, and/or
2. aromatic ring carbons, and/or
3. carbonyl carbons $\left(C=O\right)$

Since the reaction sequence itself is unavailable, I cannot verify this rigorously from first principles.

Therefore, based only on the provided stored answer, the numerical answer is taken as:

$\boxed{7}$