If the lecture room is usually full, is it appropriate for a TA to compete with the seat of students? There is one topic in the course that I am TAing that I felt interested to re-learn (since I forgot most things about it). I also had some questions I want to ask about the topic (that could be beneficial to students). As in the question, knowing that the room is frequently overflowing, is it not appropriate to "steal" the seat of students to sit in the lecture?
Ask the professor

The professor should be the one to make this decision, weighing the potential positive and negative impacts on the students.

As a TA, you should in any case ask the professor before planning to attend lectures, whether the course is full or not, and you should particularly ask whether it is okay for you to ask questions.  In some classes, the professor might think having a TA present would be distracting, and may or may not share your view that your questions would be "beneficial to the students".  

If you do end up asking questions, it would be good to check in with the professor periodically to see if they are still finding it helpful.  

If the professor asks you not to attend the class, then it is your responsibility to find some other way to get up to speed on the material (i.e. reading on your own).  If you do not think that will be possible, then you should ask to be assigned to a different course.