On translatewiki.net, only singular and plural are currently allowed in Interslavic [isv] translations, for example:
{{PLURAL:$1|edit|edits}}
In Interslavic, this should look as follows:
{{PLURAL:$1|pravka|pravky|pravok}}
Based on https://translatewiki.net/wiki/Plural/Mediawiki_plural_rules, it should be "Rule K" as is the case for most other Slavic languages.