How do you measure user retention in your community?

What KPIs or metrics do you track to measure user retention in your community?

We’re building a SaaS community and leadership wants us to share retention numbers at the next all-hands. The problem is… we haven’t been tracking anything yet 😅 and honestly have no idea where to start.

If you’re a community manager (ideally in SaaS), what metrics do you actually use? We’ve got about 40 days before the meeting, so we could start collecting some data now.

Any advice would be huge..thanks!