I can see only about 20-30 values in a single attribute, no more

Hi,
It is defined in a WooCommerce View:
https://plugins.trac.www.ads-software.com/browser/woocommerce/tags/7.1.0/includes/admin/meta-boxes/views/html-product-attribute.php#L41

I can imagine you could add some JavaScript to change that limit. Not sure how it all interacts though.

Changing it manually here in the code gave more then 50 results in the view. Ofcourse changes in the code will be overwritten on every update of WooCommerce.

You could ask this same question, now with a bit more info, in the WooCommerce support forum.