Refer to the Maths Modulo filter: http://docs.shopify.com/themes/liquid-basics/output#modulo
{% assign value = collections.size | modulo:2 %}
{% if value == 0 %}
even
{% else %}
odd
{% endif %}
Or if you'd prefer a one liner:
{{ collections.size | modulo:2 | plus:1 | pluralize:'even','odd' }}