The people running this census messed up everything. You will hear how bad this Spring. Nobody will ask you for that number – ever.
Just move on and don’t worry about it.
Live your life. The chances of anyone asking for it is pretty slim.
Do nothing. They give you a confirmation in case you need to prove that you took the census. It’s not mandatory to take the census.
There’s no law mandating completion of the US census survey. So you don’t need to worry about your confirmation number.
Nothing they won’t ask for it. You did your duty.

I did the Census online, but lost my comfirmation number. What do I do?