In other industries, perhaps, but it's standard in tech work to move jobs frequently. Every time I hire a new dev I slog through 200 - 300 resumes, the overwhelming majority of which have never held a job longer than 18 months.

There's an easy rule of thumb ... is there an upward trajectory in the responsibility or stature of the roles, or are the moves mostly lateral?

A series of quick lateral moves - this obviously excludes freelancers and consultants who specialize in short term contracts - can be a sign of someone who stays just long enough to either show they're not great at the role and took off before they were let go, or otherwise has something else going on with them that you'd want to look into more. It could be perfectly understandable. It can often not be.

Eh, not at the moment. It's a sellers market. Maybe if a recession comes about.

It's not the total number but the frequency. If your average tenure is around two years then you should be totally fine. Anything between 1-2 years might be fine or might raise some questions. Below one year, then yeah, it might start looking bad.

If you think the total number is a problem, just start dropping the oldest jobs off your resume, and don't list university graduation years.

Maybe. But every time I've changed jobs my pay increased 20 or 30k.

That means I'm doing higher demand work.

So at least my most recent skills are extremely valuable.

That definitely has not been the case in my observation, including my own job hopping career.