You can use a Cloud Tasks queue to regulate the rate of Workflows executions.

For Workflows, there is a maximum number of active workflow executions that can happen concurrently. After the limit is reached, new executions fail with an HTTP 429 Too many requests status code. By enabling a Cloud Tasks queue to execute child workflows at a rate that you define, you can avoid Workflows quota-related issues and achieve a better execution rate.

For instructions on how to do this, see the full tutorial in the Workflows documentation.