https://github.com/jenkinsci/kubernetes-plugin/blob/master/src/main/java/org/csanchez/jenkins/plugins/kubernetes/KubernetesCloud.java#L788 :

    private boolean addProvisionedSlave(@Nonnull PodTemplate template, @CheckForNull Label label) throws Exception {
        if (containerCap == 0) {
            return true;
        }

But a few lines later, the code checks if the instanceCap of the containerTemplate is maxed out.

So if you set the global container Template to 0 (which btw. is not documented to mean "unlimited"), your instanceCap (which is completely undocumented), is ignored, which is also not documented AFAIK.

Either this behavior should be documented (maybe documenting instanceCap on the way), or fixed.