One is likely sufficient if you only want to spend or receive funds. And over time you may want to do both. However, one channel gives little redundancy. If your channel partner goes down for an update or because of technical issues you will not be able to send or receive for the that period of time. It's recommended that you open at least 2 or 3 channels as minimum.
If you're looking to help the Bitcoin Lightning Network community or want to earn fees from routing the more channels the better.
