Singular:
Migration: %1$d Legacy Ignored Post was migrated but %2$d failed. To see the migrated event you will first need to refresh this screen. Migration: %1$d Legacy Ignored Post was migrated but %2$d failed. To see the migrated event you will first need to refresh this screen.

Plural:
Migration: %1$d Legacy Ignored Posts were migrated but %2$d failed. To see the migrated events you will first need to refresh this screen. Migration: %1$d Legacy Ignored Posts were migrated but %2$d failed. To see the migrated events you will first need to refresh this screen.

This plural form is used for numbers like: 0, 1

This plural form is used for numbers like: 2, 3, 4