How can I get numbers on my iPad?

The tags appended to your post suggest that you have first generation iPad Air. This model of iPad can only be updated to iOS 12.4.8.

Apple ended major update support for iPad Air1 in September 2019. An iPad Air1 cannot be updated to iPadOS 13 (or any later major versions of iPadOS) as the internal hardware does not meet the minimum technical requirements (RAM and CPU) for newer versions of iOS/iPadOS.

The Apple productivity Apps for iPad, such as Numbers, now requires iPadOS 13.1as a minimum. As such, these Apps are not available for iPad models that cannot support iPadOS13/14.

Whilst many App titles remain compatible with your iPad Air1, if you want or need to access to Apps requiring the most recent versions of iOS, you’ll need to consider replacement of your iPad with a newer or current model that supports iPadOS14.

Sorry, I appreciate that this may not be what you were hoping to learn if you do, indeed, possess an iPad Air1.

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

Which iPad do you have?

You are correct numbers need iOS 13.1 or later.

If your iPad can not run iOS 13, then you can not get numbers, you would need to buy a new iPad