AEM can be hosted in multiple ways.
1. AEM as a cloud service (Fully managed Adobe SaaS). Product description is available at https://helpx.adobe.com/uk/legal/product-descriptions/aem-cloud-service.html
2. Adobe Managed services where Adobe do support but not fully SaaS - product description is available at https://helpx.adobe.com/uk/legal/product-descriptions/adobe-experience-manager-managed-services.html
3. On prem hosted by the customer itself. Product description at https://helpx.adobe.com/uk/legal/product-descriptions/adobe-experience-manager-on-premise.html
Each of the above options specifies the various specifications.
To answer in one line, its close to unlimited but constrained by the licensing terms.
