(9 May 2012)
I have more than 10 total cores (ie multiple sockets, cores, and/or hyperthreading) but my CPU graph only goes to 1000 (ie 10 cores).
Older (ie today I have 1.2.5-2.el5.rf from RPMforge) versions of munin-node had a cpu-calculation module that calculated CPUs like this:
NCPU=`expr \`grep '^cpu. ' /proc/stat | wc -l\` - 1`
The problem is that if you had more than 10 CPUs, you ended up with entries like cpu10
in /proc/stat which don't get matched by that regexp.
So the solution is to upgrade to a more modern version of munin.
If that isn't possible/practical, you can always modify the cpu plugin to calculate the value differently. After some playing around, I modified my plugin to calculate the CPU number as so:
NCPU=`expr \`grep '^processor' /proc/cpuinfo | wc -l\``