This should do it for you - all your files will end up called Part1-Part500.
#!/bin/bash
FILENAME=10000.csv
HDR=$(head -1 $FILENAME) # Pick up CSV header line to apply to each file
split -l 20 $FILENAME xyz # Split the file into chunks of 20 lines each
n=1
for f in xyz* # Go through all newly created chunks
do
echo $HDR > Part${n} # Write out header to new file called "Part(n)"
cat $f >> Part${n} # Add in the 20 lines from the "split" command
rm $f # Remove temporary file
((n++)) # Increment name of output part
done