What is the maximum regeneration current on a Harmony BMS?

It mainly depends on what the battery cells can handle rather than the BMS.  The BMS itself can take the same regen current as output current as it goes through the main power path.

Through the charging port it can only go up to the specified charge current.

Charge current, battery current, and battery regeneration current can all be individually configured using VESC Tool to suit your specific application, hardware or performance requirements.