Where can I enter number fo qualifying children taken into account in determining the AdvCTC?

You enter your dependents in My Info.

Make sure you have entered your child as a dependent in My Info, and that you have entered the child's Social Security number. If your child was born in 2021make sure you said he lived with you the whole year.  There is an oddly worded question that asks if the child paid over half their own support.  Say NO to that question.

Federal>Deductions and Credits>You and Your Family>Child Tax Credit

The IRS is sending out letter 6419 to you.  It will show the amount of advance child tax credit that you received during 2021.  Enter the information from that letter carefully.   

The remaining amount of CTC that you can receive will show up on line 28 of your 2021 Form 1040.   Then it will calculate down to the refund you see on line 35a or the tax due on line 37.

PREVIEW 1040

https://ttlc.intuit.com/questions/1901539-how-do-i-preview-my-turbotax-online-return-before-filing

Click on Tax Tools on the left side of the screen. Click on Tools. Click on View Tax Summary. Click on Preview my 1040 on the left side of the screen.

NOTE:   The CTC is indeed a “credit” that can be applied toward any tax liability that you would otherwise have to pay as “tax due”  to the IRS.

On your Form 1040 in the Dependents section

You can view your Form 1040 at any time using the online editions. Click on Tax Tools on the left side of the online program screen. Click on Tools. Click on View Tax Summary. Click on Preview my 1040 on the left side of the screen.