Remarks or Remark?

Yes, remarks is the plural of remark.

Now, in the described context, you usually use remarks.

Why the plural? Because you want to invite the user to include all remarks he wants to share! If you use the singular, you are basically saying that only one remark is expected / permitted.

A remark could be: "Nice questions." Another remark could be "Question 12 was not relevant to my situation."

Now, presented with a text-box labelled remark, I might feel I should chose one of the two remarks, whereas you probably want to have both.

So you include one text field labelled remarks, or you present a variable number of text fields labelled remark.