They're applied to the caller, so "in total" in this instance. They're pretty high, however -- 300 write requests per minute per GCP project.
That's 5 per second over all your customers? Not too worried about reaching that quickly?
Not particularly. A large portion of our customers who sync data to Google Sheets use a daily frequency, so the theoretical upper limit is close to a half million sheets being written to (per GCP project). We have other projects available that we can start using once this gets close to becoming an issue.
That's more reassuring indeed.
Cool project, good luck!
At my company we routinely write to hundreds/thousands of sheets a minute, you can get much higher quotas if you ask.