Generally speaking, six or more. Unless it's none at all.
The number of channels a hearing aid has is often a simplistic way an audiologist will use to explain why one hearing aid is better than another, but channels are complex and it is really not that straightforward.
Hearing aids amplify sounds of different frequencies by different amounts. Most people have lost more high frequencies than low and therefore need more amplification in the high frequencies. The range of sounds you hear are split into frequency bands or channels and the hearing aids are set to provide the right amount of hearing at each frequency level.
Less than six channels and this cannot be done with much accuracy, so six is the magic number. However, a six channel aid is typically very basic with few other features and is suitable only for hearing a single speaker in a quiet room. The number of channels is not what you should be looking at, it's more the rest of the technology that comes with them.
As a final note, different manufacturers have different approaches. One method is not necessarily better than any other. For example some manufacturers have as many as 64 channels in their top aids. Most tend to have between 17 and 20. One manufacturer has no channels at all.