A mapping I added to my .vimrc based on the answers here:

```
" count nr of occurrences of word under cursor
nnoremap <leader>c :%s/<c-r><c-w>//gn<cr>

" count nr of occurrences of visual selection
vnoremap <leader>c :<c-u>%s/<c-r>*//gn<cr>
```

A bit of explanation, hopefully helpful for newer vimmers:  

 - `<c-r><c-w>` inserts the word under the cursor in the command line,
   super handy in many occasions.  
   
 - The `<c-u>` is needed to remove the automatically inserted `'<,'>`
   when pressing `:` in visual mode and going to the command line. The
   `*` register contains the (last) visual selection, so `<c-r>*`
   inserts the contents of the register in the command line (can also be
   used in insert mode).