There are six, three in each sense, depending on where you start modulo-3.
This term is useful because:
- there are some crazy constructs, notably in viruses, in which there's more than one gene in a single orf
- post-transcriptional modifications can throw out parts of the sequence
NCBI online tool to find and view all open reading frames in a given FASTA: www.ncbi.nlm.nih.gov/orffinder/