It would seem to make sense for the number to be at least that of the highest number allowed free by a site that eBid allows import from. That would do away with any incident of people importing listings to eBid and then getting hit with unexpected fees (particularly as Ninja Lister and the spreadsheet give no warning of fees).
As said on another thread, now image storage is cloud based the cost to eBid for increased image storage should be minimal or even zero to what they now pay.