(1) No amount of components in the base library will fit every requirement "out of the box"
Every customer is different, and every UI is different. In SCORE, components are designed to be nested, restricted, and styled. But there will always be cases where variants of components are needed, or simply new components are required to satisfy a UX requirement or a functional requirement from a customer.
(2) HTML is CHEAP
For the most part, building components should be cheap. UX elements are not meant to house sophisticated, complex logic, rather they should be simple presentation elements fed some data or content to render. By cheap, we mean that new components should not cost a lot to engineer.
That being said, you have to decide what the correct number of components are for your user.
(3) Component design is a trade off between granularity and composition
Making components small and granular increases the flexibility given to the user, but it also increases the styling requirements and the number of items the user must utilize to build the page.
(4) Components must be portable
Portability of components means several things about the component should be flexible - including styling of the component, and datasource locations.