You can set the store credit/refund limit in the General Configuration Settings of the Magento 2 Store Credit Extension.

Read this How to Configure Store Credit Extension in Magento 2 Store to setup this extension.

Here are the few more resource to help you get most out of this Magento 2 Store Credit Extension.

• Maximizing revenue with Magento 2 Store Credit Extension
• Raise Loyalty & Returning Visitors with Magento 2 Store Credit Extension
• Store Credit in eCommerce: A Key to Customer Retention