Hi,
from your output above:
4815 free pages
each page is 4k ... ok ok 19 MB
Still by far not enough for oracle to operate seamlessly. Once your free list goes to 0 what will very likely happen during backups or batches, the DB stops working until there are again a few pages free. Depending on the size and load on the database we try to keep the free list 6 digits if possible ... on high transactional huge databases even bigger - to have enough headroom to operate.
Kind regards
zxmaus