Siman 35
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א נָהֲגוּ בְּמִנְיַן הַשִּׁטִּין לִכְתֹּב בְּשֶׁל יָד שִׁבְעָה שִׁטִּין בְּכָל פָּרָשָׁה וּבְשֶׁל רֹאשׁ אַרְבָּעָה שִׁטִּין, וְאִם שִׁנָּה לֹא פָּסַל:
1 The 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.