I need to know how to get my AIG number from 2018 taxes when I use TurboTax

You need your 2018 AGI to efile 2019. The AGI is on 2018 1040 line 7.
https://ttlc.intuit.com/community/agi/help/how-do-i-find-last-year-s-agi/01/25947

Or see the screen shot by critter on this one,
https://ttlc.intuit.com/community/taxes/discussion/re-i-m-order-to-complete-my-taxes-it-is-asking-me-for-a-form-from-2018-that-i-don-t-have-and-i-don-t/01/995184

I need my aig number from 2018 so I can fix my 2019 taxes 

Your 2018 AGI is on form 1040 line 7. To access your 2018 return,

1. Sign in with the same user ID you used last year.
2. Click Tax Tools on the left menu 
3. Click print center. 

If you can't access your 2018 return, you can request an online transcript or a copy by mail from the IRS website.