Lokalise supports keys with plural forms. Please note, the file format you are working with must also support plurals so that they are exported properly. In the case that the file format does not support plural forms, they will be encoded as a JSON array in the translation field.
Enabling plural forms for keys
While in the Lokalise editor, click any key name to open the key editor (make sure you have key editing enabled in project settings). Navigate to the Advanced tab and enable Plural switch.
Lokalise automatically handles the plural forms according to Unicode rules, which means that some languages will have up to 6 forms and some only a few, e.g. English requires just ONE and OTHER forms.
Using custom plural forms
In rare cases, you may need to adjust plural forms. This is done per language. In the editor or in the project dashboard, click the language icon and select Language settings.
Enable Custom plural forms, select what is required and Save.
Here is the video tutorial:
If you are disabling a particular form in the language, it will remove existing translations of this plural form in all keys.