POEditor currently supports pluralization for the following localization files:
If you import a file that contains strings with plurals enabled, they will automatically show in the POEditor interface, in accordance to the language in which you're localizing the software.
You can also manually enable pluralization for a term, by accessing Terms in the Project page, hovering over the term, and pressing the P button. The actual plural forms are visibile only in the translation pages.
A term with pluralization enabled will have a separate tab for each plural form above the translation box. Here's an Arabic localization page:
In cases such as that of Chinese localization, when the localized language only has one plural form, POEditor displays the translation boxes without any plural tabs.
You can read more about plural rules at the Unicode Common Locale Data Repository: http://cldr.unicode.org/index/cldr-spec/plural-rules