It is easy to design databases with 5-6 tables. But when the table number grows it becomes very hard and tricky to define everything like relations, indexes, deciding whether to create more tables or not, how to make it scalable, etc.

I'm currently creating a database that is going to have at least 15 tablets, at least for now but it is becoming harder and harder to design it as I think more about the application flow and features.

So how to approach this? Can you guys please suggest any guides or books or something?