I just went through Word's preferences and could find nothing that would allow you to define or change the number of entries appearing in the sidebar. Although, I would note that Word highlights all of the matches in the document, even if there are 1000.
My supposition is that Microsoft probably felt that 100 was adequate to help most people find something(s) obscure in a document, but if there were more matches than that, it probably just made more sense to highlight inline than bog down the system with duplicate content in a sidebar. Functionally, you should still be able to replace all the entries if you want to, or step through using the Find Next feature.