Purchase, form submit, booking, key-page view. Five or fewer; everything else is vanity.
Admin → Events. Mark existing ones (form_submit, purchase) as key events — no new code needed.
Use event snippets or Google Tag Manager triggers for anything not auto-collected.
The star toggle in Events. This is what feeds the Conversions report and bid strategies.
Trigger each conversion yourself and watch it land in DebugView before trusting the reports.