By default, Codeat’s Glossary will let you reach/show the “letter archives” by the single first initial or by a number.
For example, https://YOURSITE.EXT/glossary/?az=a will show you a page containing all the terms of your glossary that begin with the letter A.
Likewise, https://codeat.co/glossary/glossary/?az=7 will show you a page containing all the terms of your glossary identified by the number 7.
This functionality, for example, will allow you to create menu entries for each single “letter archive” through the use of the “custom link” functionality of WordPress menus.
On another case, these same URLs are automatically used by the PRO shortcode
to link the single letters to the relative archive pages (see the documentation).