Unfortunately, it does not solve the problem.
* With Storefront, the number of products per page seems to be set to 16. Both attribute pages (for which I gave the url in my first message) display 16 products
* I activated Legenda theme (and not my child theme). The issue was back (28 products on one attribute page, 30 on the other)
* I desactivated all plugins but Woocommerce (and I kept the Legenda theme). The issue remains…
Can you please give it a look?