Here is a quick example on how you can set up a run script in Xcode to auto-increment your build number.
Go to the target in the navigator window, and choose build phase. Add a build phase, and add run script.
Add this simple bash script which reads the info.plist, adds 1 to bundle version in decimal then saves it.
bN=$(/usr/libexec/PlistBuddy -c "Print CFBundleVersion" "$INFOPLIST_FILE")
bN=$((bN += 1))
bN=$(printf "%d" $bN)
/usr/libexec/PlistBuddy -c "Set :CFBundleVersion $bN" "$INFOPLIST_FILE"
Modify the script to adhere to your build numbering convention.