How do G729 Encoders and Decoders work?

Each Digium G729 channel is composed of one encoder and one decoder, when your asterisk-based system is transcoding into another codec, it will use one encoder to transform audio from the other end point codec into G729 and one encoder to transfer from G729 to the other endpoint codec. 

it's important to mention that if Asterisk is not transcoding the codec (G729 into another coder Or any codec into G729) your G729 channels won't be used. This is known as pass-through.

How many G729 channels should I buy for my PBX?

Assuming that you are not recording or spying on the call, you phone will use one channel (One encoder / Decoder) per call. If you are recording calls, your channel per call could go up to 3 channels. 

We suggest you purchase as many channels as you expect to have concurrent calls and an additional 10% -30% for overhead purposes.  As an example: if you have a PBX with 20 phones and you don't expect to have more than five calls at the same time. Your G729 license should have 7 (5 + 2) channels.