Why is contrast of key more important than the number of themes in sonata form?

In sonata form, contrast of key is of primary structural importance because it establishes the fundamental tonal opposition that defines the movement’s architecture, whereas the number of themes is of secondary significance.1

While the term "sonata form" often leads to a focus on the "first subject" and "second subject," these labels are misleading because they imply a simple contrast of melodies. In reality, the exposition is characterized by a dynamic shift from the tonic (home) key to a closely related but distinct key. This tonal opposition creates a sense of conflict that the subsequent development and recapitulation sections must strive to reconcile.1

Compared to this essential tonal design, the number of themes is flexible. A movement may feature two clearly defined themes, only one, or even more than half a dozen. While thematic differentiation can help a listener follow the tonal structure, the core of the form remains the interplay of keys. Ultimately, key serves as the fundamental articulator of form in the Classical style, with themes often functioning as mere motives that support the broader harmonic organization.12