What libraries should I consider while making site in python?
For building a website with Python, there are several libraries and frameworks you may want to consider:

Django - A high-level, full-featured web framework that makes it easy to build robust, dynamic websites. It includes features like an ORM (Object-Relational Mapping), a template engine, and a URL dispatcher.

Flask - This is a micro-web framework that provides the core functionality required to build a website, while leaving the more complex aspects to third-party libraries. It's lightweight, flexible, and easy to learn, making it a popular choice for small to medium-sized projects.

Pyramid - A web framework that can be used for a wide range of applications, from small to large-scale projects. It provides a flexible and extensible architecture, making it a great choice for complex projects with specific requirements.

You would also want to consider using libraries for specific tasks, such as authentication (e.g. Flask-Login), templating (e.g. Jinja2), and database connectivity (e.g. SQLAlchemy). 

Ultimately, you'll likely end up choosing Flask for a quick start for smaller projects and Django for more complex projects.
What might I miss out on by choosing Django or Flask compared to the other one? Will it matter if my site gets very popular?
By choosing Django, you may miss out on some of the flexibility and simplicity that Flask offers, as Django comes with a lot of built-in features and can sometimes feel monolithic in comparison. However, the trade-off is that Django provides a lot of out-of-the-box functionality that can save you time and effort in building a website. Additionally, Django is well-suited for large, complex projects and can scale well as your site grows in popularity.

On the other hand, by choosing Flask, you may miss out on some of the robust, advanced features that Django provides, but you will gain more control over the specifics of your application. Flask is lightweight and flexible, making it a good choice for small to medium-sized projects, but it may require more effort to add additional functionality as your project grows.

In summary, it depends on the size, complexity and requirements of your project. If your site is expected to handle large amounts of traffic and requires a lot of advanced functionality, Django might be a better choice. On the other hand, if you are building a simple website or prototyping, Flask might be a good place to start.