Series would only give you the series name. You need to add series index as well.
Something like:
Code:
{author_sort:sublist(0,1,&)}/{series:| |}{series_index:0>5.2f| | - }{title} - {author:sublist(0,1,&)}
One note is that this template is from my save to disk so for title use, the first bit "{author_sort:sublist(0,1,&)}/" can be removed.
The sublist item uses the first entry in the list which keeps the filenames/paths shorter when you are sending an anthology where all 20 authors are listed.
Using this template and J D. Robb's Golden in Death (In Death #50)
I get a directory called "Robb, J. D_" with files named "In Death 50.00 - Golden In Death - J. D. Robb" plus the file extension.