I have not seen any such report.
You could go to Organization/Inventory, type "MX" into the search field to only see MXs, then sort on the "Network" column.
Sites with spares will have two lines in a row. Those without will have a single row.
If you have a lot of sites you could use the "Download as CSV" option, open it in Excel, and then perform a count of every value in "Network" (only for MXs). A count of 2 means there is a spare, a count of 1 means there is not.