Run Zonal Statistics as Table. First you need to create a zone though. We will use your rainfallvolume raster to do this. Open the Raster Calculator and input Int("RainfallVolume" * 0). This will return an integer raster of all zero's (constant value) that is the exact size and shape of the rainfall data. Use this raster as the input zones using the Value field. Use the rainfall raster as the Input Value Raster, also using the Value field.
The zone we created only surrounds the real pixels. It isn't including the NoData pixels in the count. The input value raster is 897*563, which is 505,011 pixels. The count field you got indicates there is only 343,809 pixels. I ran this in the raster calculator - Con(IsNull("RainfallVolume"),1) which converts NoData to 1. I checked the count and it is 161,202. 343,809 + 161,202 = 505,011.
Get Raster Properties tool doesn't give you the product of COLUMNCOUNT and ROWCOUNT. We should probably just enhance that tool to provide TOTAL_PIXEL_COUNT. It's still currently up to you to do the math, hence my comment "Not out of the box".