One can arrive at this total by including either the תַנִין or the drowning of the Egyptians in Yam Suf.
Blood – Yam Suf
The eleven miracles from the Plague of Blood through the drowning of the Egyptians in Yam Suf all form one unit, but the תַנִין is not included.
SOURCES: Seforno Shmos 4:23 Shmos 7:4 Shmos 8:12
"אוֹתֹת וּמֹפְתִים" and "שְפָטִים" – Seforno contends that the first nine wonders were simply signs and are referred to in the verses as "אוֹתֹת וּמֹפְתִים". In contrast, the last two – the killing of the firstborn and drowning in Yam Suf – were actual plagues, and thus the Torah refers to them as "שְפָטִים".
Dual purpose of the miracles – According to Seforno, the first nine signs had an educational purpose, to inspire the Egyptians to recognize HaShem and repent. In contrast, the last two plagues were to exact retribution from the Egyptians measure for measure.
Exclusion of תַנִין – Surprisingly, Seforno does not include the תַנִין in his grouping of signs. Perhaps the omission is related to the exclusive nature of the sign. Since it was viewed only by Paroh and his staff and did not affect the rest of the Egyptian nation, it was a much less effective educational tool.
Unique character of the Plague of the Firstborn – Seforno notes that the original warning for this plague is the only one which emphasizes the concept of a measure for measure.
Inclusion of Yam Suf – As the ultimate retribution was completed only with the drowning of the Egyptians in Yam Suf, the Splitting of the Sea is also included in the count.
Literary patterns and breaks in the narrative – Seforno's count and division might be supported by the distinct literary styles of his two groupings. While the first nine plagues follow a somewhat similar pattern (the transmission of a command to Paroh to release the nation, the bringing of the plague, negotiations, the hardening of Paroh's heart and his refusal to free the people) and flow one into the other, the descriptions of the last two plagues are exceptional. Each plague is surrounded by outside narrative unrelated to the plague itself, and each focuses not just on the Egyptians but also on the Israelites.
"עוֹד נֶגַע אֶחָד אָבִיא" – This verse which precedes the Plague of the Firstborn would be interpreted as saying that there would be one more plague before the Exodus, but not that this would be the final one of the entire series.
Tanin – Firstborn
The eleven miracles from the sign of the תַנִין through the Plague of the Firstborn are all part of one series, but Yam Suf is not included.
SOURCES: Ralbag Shmos 8, Toelet 2
"אוֹתֹת וּמֹפְתִים" and "שְפָטִים" – Ralbag maintains that these three terms are all synonyms which refer to the same series of eleven miracles.
Inclusion of תַנִין – Shmos 7:9 explicitly describes the תַנִין as a "מוֹפֵת", and this warrants its inclusion. Ralbag explains that HaShem began the sequence with a relatively harmless wonder in order to gradually ratchet up the pressure on Paroh.
Purpose of the miracles and the exclusion of Yam Suf – As the primary goal of the miracles was to compel Paroh to let the nation go, this process ended once permission was granted. Ralbag views Yam Suf as a completely separate event, with the different objective of the Egyptians recognizing HaShem.
Escalating process – Ralbag delineates how these eleven miracles intensified in severity step by step.
Frame – The wonders from the תַנִין through the warning regarding the Plague of the Firstborn are framed by mention of the "אוֹתֹת וּמֹפְתִים", suggesting that all of the miracles which are in between (including the תַנִין, but not Yam Suf) are part of the same process.
"עוֹד נֶגַע אֶחָד אָבִיא" – This verse supports the notion that the Plague of the Firstborn was the culmination of all of the wonders.
