Why does it say "Mishlachas Mal'achei Ra'im"?
Radak: Mal'achei is not Samuch, just the final Mem is omitted, as if it says Mal'achim. We find like this - "Chalonei Shekufim" (Melachim I, 6:4) is like Chalonim. The Makos are Mal'achim Ra'im, for they came via Shelichus Hashem and with His desire. "Mishlachas" is something that comes in multitude, one after the other, like "Hineni Mashli'ach Becha" (Shemos 8:17).
What do we learn from the verse?
Hagadah Shel Pesach #1: Every Makah was like four Makos, for it was with Evrah, Za'am, Tzarah and Mishlachas Mal'achei Ra'im.
Hagadah Shel Pesach #2: Every Makah was like five Makos, for it was with Charon Apo, Evrah, Za'am, Tzarah and Mishlachas Mal'achei Ra'im.
Radak: This includes the Makos that were not mentioned here.
Malbim: Refer to 78:48:1:2.
