15+ and there have been a few but nothing particularly notable. Gmail has had outages too, and I couldn't tell you from personal experience which is more reliable which is interesting given the big difference in the complexity of the deployments (obviously Gmail also has the burden of much bigger scale).

I've had more Microsoft Office 365 outages than Fastmail outages in the past 10+ years, and I'm sure Microsoft has a much deeper pocket than Fastmail.

Things break. More things will break in the future due to increased complexity, brittle network automation processes and poorly written code. You can mitigate failures to a certain extent, but you can't guarantee 100% uptime, even with a triple redundant system. Every business decision is a compromise among various constraints.