HA k3s cluster without agent

According to the k3s doc, it is possible to deploy an HA k3s cluster with zero agent ( “Zero or more agent nodes that are designated to run your apps and services”)
Does this mean that in such a k3s deployment, the apps and services will run on the servers nodes?
If so, is this kind of setup recommended when the number of available hosts is small?
Or may that lead to issues (performance, robustness, bad separation between control and data…)
Thanks in advance