I recently had to run a big ingestion at work, and as it was generally a test run we didn't want to do too many at a time, so we split them into chunks of 1 million each, see here how to do it with unix.
Splitting the file
Easy just use the split command, you can also provide a suffix
split --lines=1000000 huge-file.csv --additional-suffix=.csv
Adding the header to each file
Again a nice easy one acomplished with
sed, need some more info as this one isn't as easy to read as the split, we're giving sed the pattern and the header line we can to place it in, then looking for all files starting with x that we're going to apply this to, use whatever file pattern to match your files.
sed -i '1 i your header line here' ./x*