All AIs in use are loaded, however an AI’s footprint is small since it is stateless. Also there is only ever one instance on any AI regardless of the number of agents to which it is assigned, again since the AI is stateless.
When run, only the AI(s) that a qualifier selects are executed, so if Clerk is selected, HasJob, LumberJack etc. are not executed. What you see in the visualizer is what gets run.
You can easily verify this yourself by setting break points (in code).
Well there is some overhead in evaluating the qualifiers that eventually end up at a link, so depending on your focus on reuse vs. performance, you may want to trim AIs to avoid unnecessary evaluations.
How much overhead of course depends on the complexity of the qualifier evaluations.