what is the latest safari version for my mac version 10.13.6 (17G14042)

Without upgrading the OS, 13.1.2.

With upgrading the OS, it depends on what MacBook Air model you have.

(207675)

To be specific, whether a Mac's OS can be upgraded is dependent on its year and model. Any upgrades for Safari are installed as part of the OS upgrade. Safari normally cannot be individually installed. If your Mac's OS can be upgraded, Safari will be upgraded as well to the version that OS carries.