How to translate with pluralization in Twig?

I would use a solution like this:

messages.en.xliff:

<trans-unit id="1">
    <source>some.translation.key</source>
    <target>{0} no.attendee|{1} one attendee|{2} two attendees|{3} three attendees|]3,Inf] many attendees</target>
</trans-unit>

Twig template:

{{ 'some.translation.key'|transchoice(count) }}

If you need to put some arguments, you should pass them as second argument.

Here's the prototype of the filter:

public function transchoice($message, $count, array $arguments = array(), $domain = "messages", $locale = null)

Found this from Symfony Documentation:

Symfony2 provides specialized Twig tags (trans and transchoice) to help with message translation of static blocks of text:

{% trans %}Hello %name%{% endtrans %}

{% transchoice count %}

{0} There are no apples|{1} There is one apple|]1,Inf] There are %count% apples

{% endtranschoice %}

The transchoice tag automatically gets the %count% variable from the current context and passes it to the translator. This mechanism only works when you use a placeholder following the %var% pattern.