While we have a few i18n messages that exclusively refer to 1 recovery code (for example, when you need to "enter a recovery code"), we have many messages that may be hard coded to use the plural version.

We have $wgOATHRecoveryCodesCount, defaulting to 10, but this could be set to 1 by a sysadmin...

We should make sure all references to "recovery codes" have proper plural support when there may be 1 or more, depending on site config.