The _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). When $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 artÃculos
The printf then echos this string with the %s replaced by the second argument, $count (which is the second instance of $count`) .