On the other hand, why not both?
ותעל הצפרדע. צְפַרֵדַּע אַחַת הָיְתָה וְהָיוּ מַכִּין אוֹתָהּ וְהִיא מַתֶּזֶת נְחִילִים נְחִילִים, זֶהוּ מִדְרָשׁוֹ (סנהדרין ס"ז). וּפְשׁוּטוֹ יֵשׁ לוֹמַר, שֵׁרוּץ הַצְפַרְדְּעִים קוֹרֵא לְשׁוֹן יְחִידוּת; וְכֵן וַתְּהִי הַכִּנָּם – הָרְחִישָׁה, פדולייר"א בְּלַעַז, וְאַף ותעל הצפרדע – גרינולייר"א בְּלַעַז: ותעל הצפרדע
AND THE FROGS (lit. frog) CAME UP — Really there was only one frog, but when they struck at it, it was split into many swarms. This is a Midrashic explanation of the usage of the singular noun here (cf. Sanhedrin 67b; Exodus Rabbah 10:4). But as a literal explanation one must say that the swarm of the frogs is denoted by the singular form. Similar is, (v. 14) “and there was the כנם” — the swarm of insects, in old French pedulier; i. e. a swarm of lice. So, too, here, ותעל הצפרדע means: and there came up a grenouillière (old French) i. e. a swarm of frogs.