You can make a list of the raster names you wish to add together, then step through the list, adding each one to an output raster.
from arcpy import env
# Set the current workspace
env.workspace = "C:/Data/DEMS"
#Check out ArcGIS Spatial Analyst extension license
# Get a list of ESRI GRIDs from the workspace and print
rasterList = arcpy.ListRasters("*", "GRID")
#Step through the list of raster names
for rasname in rasterList:
#Cast rasname as raster before adding
listras = Raster(rasname)
#Add each raster to an output raster.
#The previous step of casting as a Raster will invoke
#the Spatial Analyst Addition function.
outras += listras
#Save output Raster