How to Create SAAS multi-tenancy ERP Application ?

Have you tried doing a quick search about it? You need to understand the core foundation of ASP.NET MVC, SaaS, APIs and Security first before you jump into creating the application. Once you get a good grasp on how each technology works, then you can start building the application that you want. Here's one I found at google search that may help you get started: https://www.appseconnect.com/how-to-design-a-multi-tenant-application-with-asp-net-mvc/