Why "[how] many"?
I don't know the answer precisely, but I can give some thoughts
One of the meanings of how in English is "to what degree?"; so, not just "how many?", but also "how far?", "how quickly?"
How and where both have their origins in case-marked forms of the Germanic interrogative pronoun, that underlies what (and who).
- I don't find it surprising that the ranges of meanings that these forms have acquired do not coincide in all Germanic languages.
Note that in English where also functions as the combining form of what: "wherefore", "whereupon" etc. This is identically true mutatis mutandis for German ("wovon", "woaus"), and I suspect for other Germanic languages, though I don't know. This surely has its origin in "where-" as an oblique case of "what".
Hvordan is transparently a compound of hvor and dan, and I suspect its origin is just like "wherefore" and "woaus" in English and German.
It's always been this way in English:
On hu manegum wisum is godes weorc?
In how many ways is God's work?
Ælfric's Anglo-Saxon translation of Interrogationes in Old English Literature and the Old Testament
There are other OE expressions that also mean "how many/much", like hu fela, hu micel, hu swiþe.
It's worth noting that who, what, where, why, and how all come from the same source ultimately: Proto-Germanic *hwaz, which is from PIE *kʷos, *kʷis.