I received a W2 and then later a corrected W2 (W2c) that only changed one entry box on the original W2 (Box 1). How is this handled in Turbotax?

If you already filed your return you will need to wait to see if it was rejected or accepted. If they were rejected you are able to login to your return to fix what you need to fix and fix the errors that caused the rejection and refile the return. If your returns were accepted, then you will need to file an amended return. You should not try to amend your return until the original return it has been fully processed and you have received your refund or your payment has cleared.
Amended returns do not “catch up” to your original return and replace them; they are processed as two separate returns.  
Amended returns have to be printed and filed by mail.  It can take the IRS up to 12 weeks or longer to process them.   If you are due a larger refund than on your original return, your amended return should only show the difference and you will receive a separate check for it.  If you owe money on your amended return, it will show only the new amount owed, you will have to mail a check with the return. You can check the status of your amended return by clicking here: https://www.irs.gov/filing/individuals/amended-returns-form-1040-x-/wheres-my-amended-return-1

 Click here for amending instructions: https://ttlc.intuit.com/replies/3288565

If you have already filed your tax return and it has been accepted, you will have to amend your return to change the box 1 entry on your W-2. Since box 1 is the amount of taxable income, it must be correct on your final tax return. An amended return is form 1040X which must be completed, printed, signed and mailed. It can't be efiled. Do no change your original return until you begin the amended return.

Here are instructions: https://ttlc.intuit.com/questions/4667233-how-do-i-amend-a-2018-return-in-turbotax

These answers were not helpful.  I have not yet filed my return. I actually have one W2 and two W2C's. Do i combine them into one W2 in my return?  There is no W2C in Turbotax.

Why do you have two W-2Cs and only one W-2? Did you  receive a W-2C and then later another W-2C replacing the first W-2C? Whatever happened delete the original W-2 and enter the correct W-2C. If the latest W-2C is a correction to the first W-2C, enter the latest one and ignore the first W-2C.

Do we then check the box that states uncommon situations that asks if we have received a w2c even if we made corrections?

If you received a W-2c then you check the box that you received it and then correct the entries on the original W-2 rather than also entering the original W-2.