If I amend my 2018 tax return will it automatically generate a 1040x to mail in?

Yes, that's what amending does, as long as you follow the correct procedure. Go to the following link for instructions to amend a 2018 tax return that you filed with TurboTax Online.

How do I amend my 2018 TurboTax Online return?

When you amend a tax return, the amended return is prepared on a Form 1040X--if you follow the directions.

https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed?jump_to=answer_3288565

Yes, when amending a tax return the TurboTax program will create a Form 1040-X.

An 2018 amended tax return, Form 1040X, can only be printed, signed, dated and mailed by you to the IRS. The IRS will take up to 4 months or longer to process the amended tax return.

See this TurboTax support FAQ for amending a 2018 tax return - https://ttlc.intuit.com/community/amending/help/how-do-i-amend-a-return-in-turbotax/01/27579