As
@JKalchik stated above, there are a lot of variables that haven't been detailed in the original post, and I would add to those whether the charcoal being used is briquettes or lump, as lump generally burns hotter and faster than briquettes.
That being said,
two full chimneys of lit charcoal sounds like a lot, even for a whole bird. I think I would start off with a full charcoal ring of
unlit charcoal, and add only about a half a chimney of lit charcoal (Minon method).
As the temperature came up to between 250F & 300F, I would start closing down the bottom vents to minimum openings to level off at about 350F and smoke cook from there, but to each their own.
One last item, be careful with such a large turkey, I usually keep my WSM-cooked turkeys smaller than 15 lbs.
If you do attempt a turkey that big, take particular care to push through the meat temperature "danger zone" of 40F to 140F in less than four hours, or you risk having a turkey that could make people sick.
