Both my husband and I received a Letter 6475. Do I add the total of the two letters when asked about the amount of the third stimulus check?

Did you get stimulus payments for three people? 

4,200 would be the amount if issued for three. 

If you claimed a dependent on your 2020 return, but cannot claim  that person on your 2021 return, you most probably received 4,200 which would match the letter and which is what you should enter. 

Please check and verify what you actually received. Look at your bank statements if the payments were made direct deposit. 

Thank you for your response! You are correct - I found a $4200 deposit from the IRS on June 3. I greatly appreciate you taking the time to provide a thoughtful answer.