I found many solutions and scripts on this site, but the easiest solution to increase the build number in Xcode is:
Go to Build Phases in your TARGETS section and add Run Script Build Phase:
Change Shell to /bin/bash and insert the following script:
#!/bin/bash
buildNumber=$(/usr/libexec/PlistBuddy -c "Print CFBundleVersion" "$INFOPLIST_FILE")
buildNumber=$(($buildNumber + 1))
/usr/libexec/PlistBuddy -c "Set :CFBundleVersion $buildNumber" "$INFOPLIST_FILE"
Have fun! :)