I have about 50 scanned topo maps that I have georeferenced in ArcMap. The paper maps have UTM coordinates so I set the data frame to the appropriate UTM projected coordinate system for their zone and georeferenced them.
Once they have all been georeferenced, it is possible to reproject the scanned map rasters and change the data frame to Web Mercator? I would like to use them in ArcGIS Online with an ESRI basemap.
Also you need to define the projection system (UTM) to the scanned toposheets before georeferencing. Then Georeference, followed by reprojection of the toposheets (Web Mercator) as proposed by Dan.