some configuration options in oxid admin have tooltips, they should apper when you click on the
[ ? ] buttons next to the text input, like that:
setting min/max price makes the category ignore assigned products, and instead all the products within this price range will be automatically included into this category.
So if you want to assign particular products, you have to leave the min/max prices empty.
(some more information about category configuration can be found here: https://docs.oxid-esales.com/eshop/en/6.2/setup/categories/main-tab.html)
pagination is the part of the page where you can select the page number:
basically, you might be seing only 20 products because the shop is configured to dispaly 20 products per page, but your pagination is not there to go to the second page. You might change product sort in this category to see, if some other products appear.