It seems to take two weeks ish to get updates on to the google index, which seems fair enough as I imagine googles index of the whole internet could be rather large.However, the some changes to my sitemaps don't seem to be being picked up, and today noticed that indexed pages had gone done. The current status shows 118 pages indexed, however two days ago was at 119 with no changes to sitemap : 
google webmaster tools, site maps and indexing, number of pages indexed reduces with no new sitemap
google;google search console;sitemap;googlebot
Google will choose what they want to index regardless of your sitemap. If they don't feel a particular page of your website serves any real purpose or value, then they could choose to drop it from their index.You can see what pages on your domain are indexed by using the site operator in Google search:-site:example.comOr for specific URL's:-site:example.com/uri-of-pageIf there are any pages not indexed that you feel should be indexed, make sure they are linked to from somewhere on your website.It is common that indexed pages will fluctuate all the time, it is generally nothing to worry about.