20
Just gives you the current volume, as parsed out of Wikipedia:Wikipedia Signpost/Templates/Issue. All it is is this:
{{digits|{{String split|{{Wikipedia:Wikipedia Signpost/Templates/Issue|2}}|" "|2}}}}
For humans to read:
{{digits
|{{String split
|{{Wikipedia:Wikipedia Signpost/Templates/Issue|2}}
|" "
|2
}}
}}
See also: Wikipedia:Wikipedia Signpost/Templates/Issue/Issue.
Note: This requires the issue template to supply a string formatted like "Volume 20, Issue 1". If you change that template's output this will get messed up!