Sheet Music Direct and Sheet Music Plus each have their own policies regarding preview pages.
Sheet Music Direct shows only one preview page: the first page in your PDF.
Sheet Music Plus shows up to six pages or half of the total PDF asset, whichever is smaller.
If the asset is three, five, or seven pages, it will display two, three, or four pages respectively.
Pro Tip: You may choose to create your own watermark filter for your PDF asset. You would place those watermarked pages first in your single, continuous PDF to ensure any preview pages on SMP and SMD included your watermark.
The buyer would simply discard those watermarked pages to play from the clean copies within your PDF, and you would control what potential customers see.