How to simply add a column of numbers in Airtable?

Assuming you're looking for an automatic sequential column of:
1
2
3
etc

You could try the autonumber field: https://support.airtable.com/docs/number-based-fields-in-airtable#autonumber-fields

That page has a bunch of info on other number based fields too!

Hi @Amy1,

There is a summary bar at the top and/or bottom of each column. (The location depends on if you have grouping turned on or off.)

Just click on the summary bar and choose SUM.

For more Airtable tips & tricks like this, you may want to check out my free Airtable training course, which you can take for free by signing up for a free 30-day trial with LinkedIn Learning.

I discuss the summary bar in my course.

p.s. If your company has a budget for your project and you’d like to hire an expert Airtable consultant to help you with any of this, please feel free to contact me through my website: Airtable consulting — ScottWorld 

Thanks @ScottWorld but how do I see the summary bar? I can't see one at all....

You will need to be in a grid view, and then hover your mouse at the bottom of the column (or at the top of a group).

Try Airtable's documentation!  https://support.airtable.com/docs/using-the-summary-bar-in-airtable-views