By default CREODIAS Finder (https://finder.creodias.eu/) shows approximate number of products found after applying a filter.

Let's say you want to find products with the following criteria:

Sentinel3
startDate=2020-02-01T00:00:00Z&
completionDate=2020-02-17T23:59:59
processingLevel=LEVEL2
instrument=SL
productType=LST
timeliness=Non+Time+Critical
sortParam=startDate
sortOrder=descending

After hitting "Search"

you will see:

Where 975 is the approximate number.

If you want to get the exact number of items, you need to edit the field at the bottom-left:

by  adding the "exactCount=1&" between "search.json?" and "maxRecords":

now hit the execute query button:

and you will see the exact number: