Hi there.
There is no set limit to categories, nor is there a way to merge two Square accounts or transfer transaction histories between them.
What you can do is download all of the reports for one account and then deactivate that account. You'll want to download the reports for your records since you'll be unable to access them once you deactivate.
Once this is done, you can then create an additional location that will replace the previous separate account from your main account in the Locations Tab of your Square Dashboard. This will allow you to Manage Multiple Locations with Square, and view reporting from a single master account.
Read up on location-specific reporting in this Support Center article: Reporting and Analytics with Multiple Employees and Locations
