How can a sonata-form exposition contain one main theme or many themes?

In sonata form, the number of themes in an exposition is of minor structural significance compared to the contrast of keys, or tonal contrast. While the exposition is often described as having a "first subject" and "second subject," these terms are misleading because they imply a simple contrast of themes rather than the essential opposition of tonalities.1

A movement in sonata form can feature a wide variety of thematic arrangements:

• One theme: Some movements, such as the first movement of Haydn’s Symphony No. 85, utilize only one main theme.1
• Two themes: It is very common for a movement to have two clearly defined main themes, as seen in Mozart’s Symphony No. 41.1
• Multiple themes: Some works contain more than a half dozen strongly characterized themes, such as the first movement of Brahms’s Symphony No. 4.1

Ultimately, the structural integrity of the exposition relies on moving from the tonic key to a closely related, distinct key. Thematic differentiation serves to help the listener assimilate this tonal design, but the interplay between thematic and tonal schemes is what provides the form's characteristic tension and interest.1