According to the Shopify rules, you can add 5 target languages. Generally, you will have 6 languages: 1 original and 5 target languages.
For further information, read this article - Supporting multi-language online stores.
Currently, we have a discussion with Shopify about the opportunity to add more languages to stores. We hope that our negotiations will be successful and our users will be able to use more languages to translate their stores.
Perhaps, you will be interested in this article - Add target languages.