No Biblical Source
The "Ten Plagues" or "עֶשֶׂר מַכּוֹת" is such a famous phrase that it is frequently taken for granted and rarely questioned. But how do we know that there were "Ten Plagues"? Surprisingly, the Torah never uses either the number ten or the term "מכות" to describe the miracles in Egypt.1 Instead it refers to them as either "אוֹתֹת וּמֹפְתִים" ("signs and wonders")2 or as "שְׁפָטִים" ("acts of judgment").3 The notion of there being a total of ten plagues appears first in literature from the Second Temple period,4 and the earliest record of the phrase "עֶשֶׂר מַכּוֹת" is in Tannaitic sources.5
The traditional count of the "Ten Plagues" begins with the Plague of Blood and concludes with the Plague of the Firstborn. However, the Biblical text sets no definitive starting or ending point for the series, leaving room to question the status of the two miracles which frame the set, the sign of the תַּנִּין and the splitting of Yam Suf. Are these fundamentally different from the other so called "plagues"? After all, these also are performed by a wondrous staff in front of Paroh, involve the hardening of his heart, and teach recognition of God. Moreover, the תַּנִּין is explicitly called a "מוֹפֵת". Conversely, perhaps the Plague of the Firstborn should be omitted from the count. Structurally it is distinct from the main plague narrative, and it is the only one of the wonders referred to as a "נֶגַע".
The phrases used in the introduction to the plague narrative plays an important role in the way we think about the series.
(ג) וַאֲנִי אַקְשֶׁה אֶת לֵב פַּרְעֹה וְהִרְבֵּיתִי אֶת אֹתֹתַי וְאֶת מוֹפְתַי בְּאֶרֶץ מִצְרָיִם. (ד) וְלֹא יִשְׁמַע אֲלֵכֶם פַּרְעֹה וְנָתַתִּי אֶת יָדִי בְּמִצְרָיִם וְהוֹצֵאתִי אֶת צִבְאֹתַי אֶת עַמִּי בְנֵי יִשְׂרָאֵל מֵאֶרֶץ מִצְרַיִם בִּשְׁפָטִים גְּדֹלִים. (ה) וְיָדְעוּ מִצְרַיִם כִּי אֲנִי ה' בִּנְטֹתִי אֶת יָדִי עַל מִצְרָיִם וְהוֹצֵאתִי אֶת בְּנֵי יִשְׂרָאֵל מִתּוֹכָם. (שמות ז':ג'-ה')
(3) And I will harden Paroh's heart, and multiply my signs and my wonders in the land of Egypt. (4) But Paroh will not listen to you, and I will lay my hand on Egypt, and bring forth my armies, my people the Children of Israel, out of the land of Egypt by great judgments. (5) And the Egyptians will know that I am Hashem, when I stretch forth my hand upon Egypt, and bring out the Children of Israel from among them.
Do the terms "אוֹתֹת", "מֹפְתִים", and "שְׁפָטִים" refer to the same or to different categories of miracles? Were these actions intended primarily to be educative "signs and wonders" or punitive "plagues and judgments"?6 Once Paroh released the nation ("וְהוֹצֵאתִי אֶת בְּנֵי יִשְׂרָאֵל מִתּוֹכָם"), was the purpose of the miracles achieved? These questions directly relate to the various options for the boundaries of the series. The sign of the תַּנִּין can fit as part of a list of wonders which come to edify, but not in a list of judgments or plagues. On the other side, the drowning of the Egyptians at Yam Suf could certainly be viewed as one of Hashem's judgments, but given its occurring outside of Egypt after the Exodus had already taken place, it stands apart from the other wonders.
In light of these issues, commentators debate both the number and nature of the set of miracles or plagues. Their differing positions further affect how the series may be divided and categorized and what should be identified as its turning point and climax.7