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The Blu-ray spec supports up to 8 audio channels, irrespective of format/codec (PCM, TrueHD, DTS-HD MA, etc).
TrueHD is lossless compression technology that packs the audio data more efficiently in order to reduce storage space and transmission bandwidth, As such, it has nothing to do with the number of channels in a soundtrack. For example: if Casablanca' were to be released on Blu-ray using TrueHD, it would still be one (mono) channel. At the opposite end, there are examples of 6.1 and 7.1 TrueHD soundtracks (mentioned above).
Film makers decide how many channels are in the soundtrack. TrueHD is just used to losslessly reduce the file size.