Proven method to sort series by book number
If you use the Metadata Plugboards in Calibre you can use the series information to be added to the Author tag which then if sorted by author will order the books based on that.
I use the following format which outputs the author information as such
{series:|| }{series_index: >3s| Book | By }{author}
Jack West Jr Book 3 By Matthew Reilly
You can use this and then use the collections to group the book series and the author sort will order the books
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