If in the general case, you need to determine how many people the project may encounter and how large and complex array of data will have to work.
But it is better to start from the type of project:
Information resources: in the vast majority of cases, the answer - you do not need to plan the architecture in advance, a very small number of information resources outgrows the capabilities of a single server with a properly configured content management system.
Online stores: here it all depends on the breadth of the assortment, if the assortment theoretically cannot grow to hundreds of thousands of items, then you do not need special solutions.
Social networks: the main criterion is the theoretically possible number of participants and links between them. If we are talking about a small regional or narrowly themed social network, you can not bother to put everything in a relational database, but something larger requires a scalable solution.
Technological projects: projects based on technical know-how directly depend on it, as a rule, when designing the "zest" of the project, it becomes clear which architecture is more suitable for it and how to develop it in the future.