_n (see Codex) function accepts four arguments. The first two are texts to be translated (the first is singular, the second plural), the third argument is a number (in this case
$count is 1 the first string is used, otherwise the second string is used.
So in fact the first instance of
$count is only used to determine whether the single / plural text should be used.
_n then translates that string, using the plug-in/theme domain - i.e. the fourth argument) and the language set in the wp-config. This then returns a translated string such as:
Se han publicado %s artículos
printf then echos this string with the
%s replaced by the second argument,
$count (which is the second instance of $count`) .