If two people want to use symmetric encryption to conduct a confidential conversation, how many keys do they need?

In symmetric encryption, the same key is used for both encryption and decryption processes. This means that for two individuals to securely exchange messages, they only need one shared key. Both participants will use this single key to encrypt their messages before sending them and to decrypt the messages they receive. Since both parties need access to the same key to ensure the confidentiality of their conversation, one key is sufficient to achieve this goal.

The other options suggest needing multiple keys, which is not applicable in the symmetric key model as it operates differently compared to asymmetric encryption, where a pair of keys (public and private) is required for each participant. The necessity for three or eight keys is not relevant in this context since the fundamental principle of symmetric encryption hinges on the use of that single shared key.