You can define a ReplicaSet with fields, including:
A selector—lets you specify how the selector should choose pods.
A number of replicas—this indicates how many pods the ReplicaSet should maintain.
A pod template—the ReplicaSet uses the pod template when creating pods, and adding pods to meet the required number of replicas.
Once you define the fields, the ReplicaSet starts creating and deleting pods to reach the specified amount.