Perhaps Ctrl-6 is not recognized.
Open (neo)vim and go into insert mode. Then enter Ctrl-V followed by Ctrl-6. See what it prints.
For me on Ubuntu (GNOME-Terminal) this prints
^^. So here Ctrl-6 is Ctrl-^.
See what it prints for you, then you are able to fix your mapping.
In the way described above, you can check if a control-key is mappable.
To check if Ctrl-X is mappable, you go to insert mode and hit Ctrl-V followed by Ctrl-X. This prints
^X (this is one character). So Ctrl-X is mappable.
Now we would like to map Ctrl-#: Again Ctrl-V followed by Ctrl-#. This just prints
#. So Ctrl-# is not mappable, as it is identical to #. (Tested with GNOME-Terminal.)
Some keys might already be used as shortcut by the terminal. In that case the terminal consumes the key and does not send it to Vim. If this is the case, check if you can reconfigure shortcuts.
Note that the terminal is in control here. (Neo)Vim only get those keys, that the terminal sends to it.
Also note that gVim has more capabilities. E.g. gVim is able to distinguish between Home and kHome (NumPad). Vim in GNOME-terminal is not.