Apart from the number of pages available, there is another important factor determining the length of an index: just what is to be indexed. This is not an issue considered in advance by most authors, I suspect, but your choices here will add to readers’ perceptions about the index and your book.
Does it look as if the indexing job has been thoroughly done – but not overblown – or is this a skimpy (even sloppy) affair? Although a threadbare index useless to its purpose will immediately signify that the rest of your book is equally inadequate, more is not necessarily better. There are limits.
So what material should be indexed in your book?
Obviously, your body text must be indexed, but what about notes that comment on the text? Citations and bibliography are usually not indexed; will you follow that practice? Some publishers think it unnecessary to index a glossary (indeed, sometimes an index becomes a kind of glossary), but consider it essential to index any illustrations and captions. What does your publisher say?
With luck, your publisher will have guidelines on all of these matters but otherwise simply use your common sense.
As you can see, decisions need to be made here but they are not life and death issues, nothing even to lose sleep over. Indeed, although what you chose to index can be significant, the ultimate quality of your index depends on other factors, not least the intellectual conceptualization of the index (or mind map, discussed here) and its ‘visualization’ in the index structure, the subject of my next post.