Q: When submitting my work to an agent for consideration, how many sample chapters should I include in my proposal? –Charles C.
A: The answer is: It varies. Most agents and publishers have submission guidelines, either on their websites or in a listing resource like Writer's Market, and say exactly what you should include. Some may request one chapter, others may ask for five. Some may ask you to send only chapters that have pictures of Erik Estrada on them (though I'd recommend steering clear of those agents). Point is, to give your manuscript a chance you need to deliver exactly what these publishing gatekeepers want.
If they don't specify what they want or say something like "submit a few sample chapters," plan on sending three chapters, or 8-40 pages, depending on the type of book you're trying to sell. For example, if you're writing the book, The 100 Best Heavy Metal Albums Ever Written, chances are you're only dedicating a page or two to each album, so you'd send 3 or 4 chapters (or roughly 8 pages). But if you are writing a fascinating tome called The Life and History of Stonewall Jackson, each chapter could be 8 or more pages by itself. In that case you'd stick to three sample chapter and however many pages that entails (though try not to exceed 40).
Keep in mind that if you plan to have illustrations or diagrams or pictures in your book, you need to include some in your sample chapters as well (e.g. The 100 Best Heavy Metal Albums Ever Written better have some album artwork in your samples).
Remember, your goal is to submit enough sample content to reflect what is really in your book—your voice, your style, your story—without submitting the entire book.