Hi, any know what is the software upgrade policy (Android upgrade & security update) for Xiaomi Pad Mini?

You can pretty much get the info from google for quick answer:
Yes, Xiaomi provides regular software updates, but the frequency and duration vary by device. Newer, high-end models are promised up to four major Android OS upgrades and six years of security updates, while many mid-range and budget phones receive a minimum of two years of security patches, according to Cashify, Nokiamob, and Gizchina.com. All Xiaomi devices get regular security patches to fix vulnerabilities, with some models receiving support for longer periods, and serious vulnerabilities may be addressed even after official support ends.

Just like most of the android devices, eg my Oneplus
newer phones receive regular updates, and major OS upgrade,
older one basically stops at last major update and only receives security patch, my Oneplus 10T still receive update but not OS, only google monthly security patch.

At that price, get Samsung a series better