Thanks everyone for replying. Yes, I am talking about so-called stock-pages, i.e. this: https://www.safepub.com/product/col...page-per-10/
Finally, I got a reply from SAFE USA it's 0.4 mm total thickness. Unfortunately, they were unable to answer on my exact question: what's thickness of black foil and what's thickness of transparent foil (so I can't compare with those supplies I have already). So, it seems, the only way to understand what is it is to touch them, but these pages are not on sale in my country, so I can not just go to shop and try, I must order pack of these sheets, then pay for shipping, then pay for forwarding, wait a month, etc.
@DrewM: SAFE ships different products to different countries. You will not find Collecto in Germany or in UK. I researched it while choosing the proper collecting system for myself. They act business differently than Lighthouse and Lindner, where products are the same worldwide. https://www.safepub.com
is a USA distributor website and you can find this system there.