Thanks Xander and Martin for your suggestions.
Sorry Xander I may have not explained the issue correctly.
My ultimate goal here is to standardise a number of raster layers (696 of them). My first objective was to get the maximum value (a unique single value) for all those rasters. I've done that; but after examining the maximum value I realise that doing the mean of all the maximum values of all rasters would be a more accurate value (to later standardise them).
What Martin says is what I need to do but I'm not familiar with numpy and arrays and I'm not quite sure how to proceed. I also wonder if there is a function in scipy that would do that directly? Below is just the start of the script but as I said I don't know how to proceed.
import arcpy
import math
import os
import numpy
import scipy
from arcpy import env
arcpy.env.overwriteOutput=True
from arcpy.sa import*
arcpy.CheckOutExtension("Spatial")
env.workspace = r"D:\000_TEST_MAGNITUDE_summary\MAX_VAL_calculation\Vel_rasters.gdb
rasterlist1 = arcpy.ListRasters()
Vel_max = arcpy.GetRasterProperties_management(rasterlist1, "MAXIMUM")
Vel_array = arcpy.RasterToNumPyArray(rasterlist1)