By default, you should not to run more than 20 active workflows executions simultaneously. After hitting that limit, workflows will be queued in order not to affect performances. It does not apply to workflows waiting for a scheduled execution. In specific contexts, you may need to run more than 20 workflows, contact Adobe Customer Care to increase the limit.
You can take a look at this documentation: Workflow Best Practices which provides some information you're looking for. In addition, no of queries you can is depends on various factors like query condition that uses single/multiple tables, volume of data, and operators used within query etc. Based on the type of query you run will affect performance of your system. You can also take a look at this documentation page which talks about various considerations of how best you can use ACS data model: Data model best practices