<p>In documentation of Kubernetes IO, the <code>replicas</code> attribute is explained using itself in the definition thus it doesn't give precise information.
Does value <code>replicas: 1</code> in yaml means there is 1 pod xor that there are 2 pods: 1 original plus 1 duplicate of it? Is the default 0 or 1?</p>

Yoda
<p>As explained below <code>replicas</code> means desired number of pods.</p>

<pre><code>kubectl explain deployment.spec.replicas
KIND:     Deployment
VERSION:  apps/v1

FIELD:    replicas &lt;integer&gt;

DESCRIPTION:
     Number of desired pods. This is a pointer to distinguish between explicit
     zero and not specified. Defaults to 1.
</code></pre>

<p><a href="https://kubernetes.io/docs/reference/generated/kubernetes-api/v1.13/#deploymentspec-v1beta1-apps" rel="nofollow noreferrer">https://kubernetes.io/docs/reference/generated/kubernetes-api/v1.13/#deploymentspec-v1beta1-apps</a></p>
