@Leonardo Alberto Moreira Thank you for reaching out to Microsoft Q&A.
You cannot use the Single App service plan across the multiple/different subscriptions. you can use single app service plan across different apps in different resources groups in the same subscription which are in same region. Each app service plan can host multiple apps based on their SKU size as mentioned here.
You can also move the app from one app service plan to another app service plan across different resource group or across subscriptions as [described here].(https://learn.microsoft.com/en-us/azure/azure-resource-manager/management/move-limitations/app-service-move-limitations)
Feel free to reach back to me if you have any further questions on this.