How is usage calculated? Is there some kind of node multiplication to the minutes if multiple nodes are used in a run?

https://dagster.io/pricing

Compute time is measured in milliseconds of run-time for each step within a job, excluding time for infrastructure activation. Every billing cycle, we aggregate the total run-time across all of those steps and round down to the nearest whole minute.

We sum up the run time of each step that has occurred in a run.

Oh so in that job because I have three steps running simultaneously at the start of the dag it's like three nodes running at the same time.

yes, that’s correct