One can obtain a total of ten by including either the תַנִין or the Plague of the Firstborn.
Blood – Firstborn
The ten plagues from the Plague of Blood until the Plague of the Firstborn create a complete set.
SOURCES: Philo On the Life of Moses I, XVII (96-97), Mishna Avos 5:4, R. Yehuda in Sifre Devarim 301 and Midrash Tannaim Devarim 26, Tanchuma Vaera 3 Bo 4, most commentators
"אוֹתֹת וּמֹפְתִים" and "שְפָטִים" – This approach could either understand that all of these terms refer to the entire Ten Plagues, or that "אוֹתֹת וּמֹפְתִים" refers to the first nine plagues while "שְפָטִים" refers to the Plague of the Firstborn.
Purpose of the miracles – Philo view the Plagues as retributive and punitive in character.
Exclusion of תַנִין – As the תַנִין did not punish the Egyptians or have harmful repercussions, it is not counted as one of the Ten Plagues.
Exclusion of Yam Suf – The drowning of the Egyptians at Yam Suf is viewed as its own distinct unit because of the magnitude of the miracle, or because it occurred outside of Egypt and only after the Exodus.
Patterns and divisions – This approach can divide the first nine plagues into three triads, each of which shares a number of common elements and features.
Hardening of Paroh's heart – The Tanchuma notes that it is exactly midway through the Plagues that the verses state that HaShem begins to harden Paroh's heart (rather than Paroh himself doing the hardening).
"עוֹד נֶגַע אֶחָד אֲבִיא" – This verse supports the idea that the plagues reach their climax with the Plague of the Firstborn.
Tanin – Darkness
The ten miracles from the תַנִין through the Plague of Darkness are a distinct unit.
"אוֹתֹת וּמֹפְתִים" and "שְפָטִים" – This approach would maintain that the terms "אוֹתֹת וּמֹפְתִים" refer to the ten wonders from תַנִין to the Plague of Darkness, while the word "שְפָטִים" refers to the Plague of the Firstborn.
Purpose of the miracles – The "אוֹתֹת וּמֹפְתִים" may have been designed to lead the Egyptians to recognize HaShem, while the "שְפָטִים" were intended simply to punish or to compel Paroh to release the Israelites.
Inclusion of תַנִין – Shmos 7:9 explicitly describes the תַנִין as a "מוֹפֵת", and this warrants its inclusion in the set.
Exclusion of בכורות and "עוֹד נֶגַע אֶחָד אֲבִיא" – This approach might suggest that the first ten miracles are distinct in purpose from the Plague of the Firstborn. Only the Plague of the Firstborn is referred to as a "נֶגַע" and thus stands alone.
Literary patterns and the narrative flow – The narrative from the sign of the תַנִין through the Plague of Darkness is recounted in a similar literary style, with a recurring cycle of features present in these ten miracles.
Frame – These ten wonders are framed by an introduction where HaShem mentions the upcoming "אוֹתֹת וּמֹפְתִים" and a summary in which we are told that Moshe and Aharon executed them.
