Why "duo pueri", but "duae feminae"? Do numerals change in Latin?
Yes and no, some are declined and some are not.
unus, una, unum for "one", duo, duae, duo for "two", and tres, tria for "three" are some that are declined.
quattuor ("four") and quinque ("five") are two that are indeclinable.
Only unus, duo, and tres, are declined, not >3
Multiples of hundred are also declined: ducenti, -ae, -a; trecenti, quadringenti etc.
The words "duae feminae" ought to be read: "duE feminE" like in english words: sEven, tEn, bEst, . . .
The lectors have many other difficulties ( Marci, Marce). It ought be changed !!!
Maybe if it was an Ecclesiastical Pronunciation you would say it duE feminE but the course is using a reconstructed Classical Pronunciation, where ae is said to be pronounced more like 'eye' or the 'ai' in 'aisle'.