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.