Answer
Only 50 rasters of the ESRI GRID format can be added at one time to an ArcMap session in versions of ArcGIS prior to 9.0. At ArcGIS 9.0, this limit has been removed.
If users plan to add more than 50 grids in versions of ArcGIS prior to 9.0, change each of the grids to another format, such as TIFF. Users can add more than 50 TIFFs to an ArcMap session.
In versions of ArcGIS prior to 9.0, more than 50 grids can be displayed at one time if they are part of a raster catalog. For more details on raster catalogs, see the online help topic: ArcCatalog > Working with rasters > Working with raster catalogs.