Yes, you can add, remove or change the number of instances of a service any time.

In the following screenshot, the "hello" service has 3 instances of itself:

Only when you reach the limits of computing power and available memory in your cluster services may fail to schedule. You can then choose to upgrade your cluster by adding a new node. You can do this any time. 

Here's a screenshot of a service reaching maximum capacity of the cluster:

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