Please explain using des/de in "on saura qu’il y a des reines autant qu’il y a de femmes"

Bonjour Dino !

Mmmh, that's a tricky question.
I would say here that it's a question of fluidity and elegance. As you might know, if some cases, des can become de for elegance sake (see our lesson: https://progress.lawlessfrench.com/revision/grammar/use-de-d-instead-of-des-in-front-of-adjectives-preceding-nouns-partitive-article ), so I would assume it's a similar context here.

As a French speaker, I could also choose to say des femmes, but it would indeed sound less elegant...

I hope that's helpful :)