Description
To calculate multiband output statistics from multiband input rasters, all inputs must have an equal number of bands.
Solution
If some of your intended inputs have fewer bands than others, you need to create new NoData bands to get an equal number of bands for all inputs. You can do this using the Composite Band tool to create new multiband datasets, combining the existing bands with NoData for the missing bands. You can then use the resulting multiband rasters as inputs to the Cell Statistics tool.