Any run of published workflow, successful or failed, counts as a billable run - whether only one or multiple steps are executed. In practice, that means if a workflow was triggered and went through its steps (even if some branches or steps were skipped), it counts toward your run total as long as the workflow execution was completed (regardless if the final outcome was a success or failure).
Test runs (runs executed in test mode while you’re building or debugging a workflow) do NOT count against your usage. We only count real, published workflow executions.