Shulchan Arukh, Orach Chayim 35/שולחן ערוך·אורח חיים ל״ה

1The practice in the count of the lines [marked on the parchment to guide the scribe] is to write on the hand tefillin parchment seven lines for each parasha. For the head tefillin [the practice] is to mark four lines. If one diverged from this practice he did not invalidate the tefillin.