I just noticed a warning message stated that this function is deprecated. I did some Google searching for more information, but could find no mention of when or why this function was deprecated or what, if anything, has replaced it. I use this function to determine the number of required instances of my effect filters (one per channel). It still seems to work, even with the latest version of JUCE.
I guess this is because of the multi bus feature. getNumInputChannels is not specific about the buses. Instead you should use
getBus (true, 0).getNumChannels()
getTotalNumInputChannels() work if you call it from
Yes, when prepareToPlay is called the bus layout negotiation is already finished.