The 13th mitzvah is that we are commanded in the actions1 involving the tefillin worn on the arm.

The source of this commandment is G‑d's statement (exalted be He), "And you shall bind them for a sign upon your hand."2

This mitzvah is also repeated in the Torah four times.3

The proof that the tefillin of the head and of the hand count as two separate mitzvos is from the tractate Menachos.4 In this passage, the Sages expressed surprise at the one who proposed that one may not wear the tefillin of the head and of the hand independently unless both of them are available.5 The exact quote is, "One who doesn't have two mitzvos, shouldn't do one mitzvah?!"

The meaning of this statement: A person that does not have the possibility of performing two mitzvos shouldn't perform at least one mitzvah?! Certainly not — he should perform the mitzvah available to him, and therefore don whichever of them [i.e., tefillin of the head or of the hand] is available.

We see from this passage that tefillin of the hand and of the head are called "two mitzvos."

Women are not obligated in these two mitzvos [of tefillin]. This exemption can be seen from G‑d's statement (exalted be He) stating the reason for this obligation [of wearing tefillin], "In order that G‑d's Torah be in your mouth"6 — and women are not obligated in Torah study. This is the Mechilta's explanation.

All the details of these two mitzvos have been explained in the 4th chapter of Menachos.7