How can I have more than 16 keyboard shortcuts for Spaces (i.e., Mission Control)
Here's something I tried (on 10.9.5).  Unfortunately, it didn't work, but I'll note it here in case it inspires someone.


I noticed that enabling System Preferences > Keyboard > Shortcuts > Switch to Desktop 16 caused ~/Library/com.apple.symbolichotkeys.plist to update.
I found some information (1) (2) about that plist.
According to that second link, "Switch to Desktop 1" is action 118.
I tested enabling / disabling Switch to Desktop 16 in that System Preference pane whilst looking at the plist file, using /usr/libexec/PlistBuddy -c "Print :AppleSymbolicHotKeys" ~/Library/Preferences/com.apple.symbolichotkeys.plist and, as I expected, the "enabled" value for action 133 (=118+15) changed.
I added a value for action 134 based on action 133, which I hoped would cause ctrl-alt-6 to switch to Desktop 17: defaults write com.apple.symbolichotkeys AppleSymbolicHotKeys -dict-add 134 "{enabled = true; value = { parameters = (65535, 22, 786432); type = 'standard'; }; }"  Unfortunately this doesn't seem to have any effect on my system.
So: I think that the symbolichotkeys plist file contains a mapping from key presses to actions.  The actions include Switch to Desktop 1 through 
to Switch to Desktop 16, but there are no corresponding actions for desktops with higher numbers.  Sorry!