Are you referring to Stock Library or a CC App library? If you have not purchased any subscription and you have purchase a Credit pack, you will see the number of credits required at the time of purchase itself. Meanwhile, you may see this link for help: https://stock.adobe.com/plans.
You are seeing the total count of items in your library which will include all items in your library including brushes, swatches, and items you may have added to the library from within a Creative Cloud app. On the Adobe Stock site you will only see the Adobe Stock assets in your library. You can see all items in your libraries here: https://assets.adobe.com/libraries
Credits can be used for all assets on the Adobe Stock site. Standard images/asset are one credit and can also be licensed with a subscription. See the plans offered here: https://stock.adobe.com/plans