The [sign_up_spot_count] shortcode allows you to output the number of open spots total or within a specific category (Pro version). This shortcode was added in version 2.2.14 and is available in the Free and Pro versions of the plugin.
You can customize the HTML by overriding the template fdsus/sign-up-spot-count.php found in the plugin theme-files directory. More information on overriding plugin templates can be found at Overriding Templates in Your Theme
Here is a list of some shortcode options:
[sign_up_spot_count]▲ Outputs the number of open spots.
[sign_up_spot_count empty_spot_text="Filled"]▲ The empty_spot_text attribute allows you to override the default output of the number zero 0 if there are no open spots. Instead, when there are no open spots it will output whatever text you set here.
In this example above, if there are eight open spots, it will output 8, but if there are no open spots it will output Filled.
[sign_up_spot_count category_slug="foo,bar"]▲ (Pro Version) The category_slug attribute one or more comma-separated category slugs and will adjust the outputting number of open spots to only those in the categories specified.