Multiple plural forms in gettext()

First, your locale file needs to have the definition of plural. As you added on the question, in Polish case you may see

"Plural-Forms: nplurals=3; plural=n==1 ? 0 : n%10>=2 && n%10<=4 && (n%100<10 || n%100>=20) ? 1 : 2;\n"

or similar definition in (domain_name)-pl.po file.

Then, you need to prepare the translation for "%d Comment"/"%d Comments" in the .po file. For example,

msgid "%d Comment"
msgid_plural "%d Comments"
msgstr[0] "%d Komentarz"
msgstr[1] "%d Komentarze"
msgstr[2] "%d Komentarzy"

Please compile .po file into .mo file and place to an appropriate folder. (e.g. languages/(domain_name)-pl.mo

In your Wordpress (plugin/theme I assume) code, you may call it like this,

for ($i=1;$i<15;$i++) {
  printf(_n("%d Comment", "%d Comments", $i, "(domain_name)"), $i);echo "<br />";
}
printf(_n("%d Comment", "%d Comments", 112, "(domain_name)"), 112);echo "<br />";

then of course set the WordPress's locale to Polish, in wp-config.php,

define ('WPLANG', 'pl');

you should see the results with correct plural forms.