Splunk has configured a limit of 500 searches that can be displayed in the search dropdown. To get around this limit you will need to edit the following line in $SPLUNKHOME/lib/python2.7/site-packages/splunk/appserver/mrsparkle/controllers/view.py
searches = en.getEntities('saved/searches', namespace=app, search='is_visible=1 AND disabled=0', count=500, _with_new='1')
Raise 500 to the desired value.
Note that Splunk has advised that editing view.py is not supported and will be overwritten by any upgrade. Also be advised that there may be performance implications from raising this value.