This works for replayed (not rebuilt builds) builds. The output is the build number of the original build from which your current build was replayed from:

def getReplayCauseNumber() {
    // This function is used to access the build number of the build from which a build was replayed from

    def cause = currentBuild.rawBuild.getCause(org.jenkinsci.plugins.workflow.cps.replay.ReplayCause)

    if (cause == null){
        return null
    }
    
    def originalNum = cause.getOriginalNumber()
    echo "This build was replayed from build #${originalNum}"
    return originalNum
    
}