How to read a text file using vhdl
Not surprisingly, readline reads one line of text and stores it into the variable you confusingly named line_num. Your input file seem to have only one line of text, which starts with 1.

read is called with a line_content output argument which is a single character, so it reads the first character of the line and outputs it in line_content. That's why you only see a single 1 in the output.

You have to split your input file into multiple lines (each containing a single character). Alternatively, you can make a single call to readline and then iterate through your line_num variable by calling read on it multiple times.

In any case, rename line_num to line_buf or something similar. It looks confusing, especially side by side with line_content, which doesn't hold the line contents as its name advertises.