Hello,
one per tape, as far as I know there are no tape drives with multiple heads that can write simultaneously. This is mitigated by running multiple jobs concurrently and by spooling each one sequentially on tapes; and I've never had a problem with them.
It does support multi tape libraries perfectly though, with multiple tape drives writing or reading at the same time.
Read the manual for additional details.
Regards,
--Simone