We are planning to prepare a product \"online clothing store\" based on SaaS model.
Different cloth vendors shall register to our portal. They will get web interface
There are a few things involved to build a SAAS software.
What architecture should be used?
A SOA/Microservices architecture is highly recommended. Loosely coupled, clustered and load balanced.
Any reference system is available for consideration?
A google search could probably give examples.
Any specific design pattern, we can consider while implementing it?
There is no single design pattern. Sticking to SOA and highly decoupled subsystems will help. Consider microservices and evaluate its pros and cons for your requirement. Take a look at Explanation of Tenant Load Balancer in SaaS maturity model level 4
Web application, SAAS?
Any recommendation for database design?
There are some DB models that help SAAS softwares. Take a look at Databse architecture (single db vs client specific db) for Building Enterprise Web (RIA) application on cloud
How to make sure that portal will be scalable?
Ideally, a cloud deployment where new instances can be added on demand. Even if not on a cloud, a similar setup with routing/load balancing enabled where new instances can be added is required, similar to a private cloud.