is it a change, really? consider "ten", "dozen", "hundred", "thousand", "ten thousand", "million" before "fish"

now make them plural, as in "tens", "hundreds"

perhaps "couple" works both like "hundred" and "hundreds", and unless you force it to mean a noun rather than a quantifier adjective, it can be either. zero, a single, a couple, a hundred fish, vs a specimen, a couple, a bunch, a lot, hundreds of fish