Welcome to the community, @DTO_SCMA! :grinning_face_with_big_eyes: A formula won’t work for this because formulas only operate on the field values in a given record; i.e. the formula’s output for a given record will be based on that record only.
One way to get the count would be to apply a two-part filter: the first part filtering records where the date is after (or is on or after, depending on your needs) your desired start date, and the second using the AND operator to filter records where the date is before (or is on or before) your desired end date. After the records are filtered, you’ll see a record count in the summary bar, below the primary field.