To estimate the number of significant terms on a page that should be indexed, pick a representative sample of ten pages from your book.
On each page, count the number of important concepts, subject terms, personal names, etc. that you believe should be found in an index. Divide the total by ten to get an average. If footnotes that expand on the text are found, include terms from these in your count. If you need a separate cited authors index, multiply the average by 25%.