zencart does not (unfortunately) support DEPENDANT attributes (ie: the options offered in attribute "B" depend on what was chosen in attribute "A"),

So, to FIRST select the number of rows, which then presents that number of additional input fields, is not possible.

It is however, possible to restrict TEXT input fields to a certain number of characters.

We do it HERE.

In your case, you would create a separate TEXT attribute for each row, then just make sure you inform your customers that they need to input values into the appropriate rows.

It is also possible to add pricing to the inputs. So you can actually charge for each letter typed in.