I’ve seen two ways to include mother’s name.
1) Include “ו)שם אמה שרה ע״ה )” right before תנצב’ה, right after, or bottom left of matzeva. Saw this on the matzevos for Rav Shimon Schwab ZT”L, Rav Pinchos Hirschprung ZT”L, and Rebbetzin Esther Jungreis A”H. For example: ושם אמה שרה ע״ה . This appears to be the most common approach when mother’s name is included.
2) Include alongside father (ex __ bas Avraham v’Sara). Saw this on the matzevos for Rav Moshe Feinstein ZT”L and Rav Meir Zlotowitz ZT”L.
I can provide pictures of the matzevos mentioned if you would like. Please let me know.
Also, ת.נ.צ.ב.ה stands for תהֵא נשמתוֹ צְרוּרָה בִּצְרוֹר הַחַיִּים for an individual and is loosely translated as “May his/her soul be bound up in the bond of life”