According to Lumo:

The Enterprise‑D’s interior volume is about 5 000 779 m³, which converts to roughly 176 million ft³.

If we treat a human body as having roughly the density of water, Michael Dorn’s mass (≈ 100 kg) corresponds to a volume of about 3.5 ft³ (100 L ≈ 0.10 m³ ≈ 3.5 ft³). Using a mid‑range estimate of 3.75 ft³ per person gives a convenient round‑off for calculations.

\frac{176,000,000\ \text{ft}^3}{3.75\ \text{ft}^3/\text{person}} \approx 46.9\ \text{million people}

If we use the lower bound of 3.5 ft³ per person:

\frac{176,000,000}{3.5} \approx 50.3\ \text{million people}

So, roughly 47 – 50 million “Michael Dorn‑sized” individuals could theoretically fill the interior space of the USS Enterprise‑D.

(Of course, this is a purely geometric exercise—real‑world considerations such as furniture, equipment, structural bulkheads, air, and the need for circulation would drastically reduce any practical capacity.)