There is a setting to control the number of object to show at a specific zoom level, this is what you change with the "More deep sky" button.
You can reset it or manage more finely using the menu Setup / Chart,coordinates then the tab Object filter.
Also remember that what you set here is limited by the catalogs currently in use.
Now the number of object on the chart that receive a label is also limited to prevent too much are displayed and overload the chart.
This is done by setting how brighter the object must be above the magnitude limit of the chart. For DSO the default is to plot a label for objects 4 magnitude brighter than the limit.
You can change this value from the menu Setup / Display at the tab Label, in column "Magnitude difference".
For example with the default value and a FOV of 20° it show DSO to magnitude 15 and plot a label for those brighter than 11.
But there is a special rule to always show the Messier label so there is some DSO label on the wider FOV.
Edited by Patrick Chevalley, 23 June 2021 - 03:10 AM.