It's really hard to put a number on the amt of candles (or any other reagent) per raid, as it depends on the kind of night we are having. There have been times I've gone through less than a stack (20) and then there are nights like last night.. where I burned through almost 40. Molox had asked that all priests bring 60 candles with them to MC, that may seem a bit overboard and I for one use to only carry 40, but there's been a time or two I've been very glad to have that 3rd stack. There is also the factor of how many priests are in the raid and what they can buff, not all priests are speced with the spirit buff ability. I can remember a time where I was the only one that could so that meant more reagents for me.
On a flawless 3 hour run the break down would be..
Candles x 24 for Fort
Candles x 24 for spirit
Candles x 24 for Shadow protection
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Total 72