The Shopify Admin API now supports bulk discount code creation. With the introduction of bulk discounts, we will allow a single price rule to be associated with multiple discount codes. Therefore, an app that wishes to create multiple discount codes for a single price rule can perform additional independent API calls for each discount code they wish to create.
This is a companion discussion topic for the original entry at https://shopify.dev/changelog/the-pricerule-resource-now-supports-bulk-discount-code-creation