How custom - good question!
I have spent half my career in ERP and the concept of "how custom?" is a perpetual bone of contention between customers and ERP vendors. This is not only the case with old monolithic ERPs like SAP but also with modern ones. In https://gtm360.com/blog/2020/05/27/when-erp-projects-get-derailed-by-silly-reasons/, I give three examples, including one of SAP-Lidl, where customer felt the extent of customizations was minor, vendor agreed to implement them, and the movies had a sad ending.