Where can I find a list of all SAP Ariba APIs?

You can access a list of all available APIs by logging into the Ariba Developer Portal and clicking the Discover icon (binoculars) in the upper-left part of the page

See SAP Ariba APIs for the official documentation on each of the available SAP Ariba APIs.

Core Procurement > Procurement REST APIs
Procurement Application Services > API Portal & Platform > API Developer Portal
SAP Business Network for Procurement & Supply Chain > Administration (Buyer & Supplier Account Administration)
SAP Business Network for Procurement & Supply Chain > SAP Business Network REST APIs
Strategic Contracts > Strategic Contracts - REST API
Strategic Sourcing > Strategic Sourcing - REST API
Supplier Lifecycle & Performance > Supplier Lifecycle & Performance - Open API