I am using 12.5 software updated iPad Air but unable to upgrade iOS 14/15 because of that unable to use Numbers App

The first generation iPad Air, iPad mini2 and mini3 can only be updated to iOS 12.5.5. The Numbers App, as you observe, requires iPadOS14 or later.

Apple ended major update support for these models of iPad in September 2019 - although they continued to receive “point” security updates until late last year. These devices cannot be updated to iPadOS 13 (or any later major versions of iPadOS) as the internal hardware does not meet the minimum technical requirements (CPU capabilities and available RAM) for newer versions of iOS/iPadOS.

Some App developers have dropped support for iOS versions preceding iOS 13.x for technical reasons (often because APIs or software libraries upon which they rely have been updated in the new version of iPadOS, making their App incompatible with earlier versions). 

Whilst many App titles remain compatible with your iPad, if you want or need to access to Apps requiring the most recent versions of iOS than your iPad is capable of support, you’ll need to consider replacement of your iPad with a newer or current model that supports iPadOS15.

I hope this information provides clarity as to your available options - and the fundamental limitations of your iPad.