How do I answer the 2020 AGI tax return question when I was a stay at home mom in 2020 and now divorced. I was on my ex husbands tax return as a dependent

I assume he filed a Joint 2020 return with you.  There is only 1 AGI on a joint return.  You each use the same amount.  If you don't have a copy of the 2020 return you will have to ask him or get it from the IRS.

You need your 2020 AGI to efile 2021. The AGI is on 2020 1040 line 11. If you filed a Joint return you use the same AGI for each spouse.

How to find the AGI.
https://ttlc.intuit.com/community/agi/help/how-do-i-find-last-year-s-agi/01/25947

If you didn't file 2020 or filed it later in the year or mailed it, then try entering 0 for the AGI. If that doesn't work you will have to print and mail your return.

Or request a transcript from the IRS
https://www.irs.gov/individuals/get-transcript

When did you get divorced?  If you were still married in 2020 he could not claim you as a dependent.  He probably just filed a Joint return with you.  

If you did not file a joint return, your AGI is zero.