From the user search docs, for the database search engine:
Regular expressions may not be used. A value of * will match all records.
For the elasticsearch search engine, you are limited to 10,000 records returned due to this bug: https://github.com/FusionAuth/fusionauth-issues/issues/494
Here's docs on how to switch between them: https://fusionauth.io/docs/v1/tech/tutorials/switch-search-engines