The limit of 10 lines is simply my choice to make it more robust and not to clog execution with redundant amounts of data. I don’t see a real reason where someone could use more than 10 lines of text, especially for dynamic social media posts. Its unlikely to use text appearances with loads of paragraph texts, which might go beyond.
Each line can contain as many characters as needed.
In the gif below, you can see that once the number of chars per line reaches certain threshold ratio, whole phrase scales down (its adjustable). Meaning, it also allows user to avoid having text reaching outside the boundaries of the comp size – which is super useful for adding phrases without worrying of extra size/lines tweaking. Also, growing of lines is possible from all 9 locations (top, center,bottom, left, center, right), combined cases.

Find out more:
After Effects Tutorials: motion design, expressions, scripting.
Boxer – Dynamic Text Boxes Template with a Live Preview
Social Media Profiles