Many people have questions about this form when filing for an immediate relative. It is very important that this form is filled out accurately and completely.
The amount of people in the US citizen's household needs to be answered honestly. If you are claiming people as dependents on your tax returns, these people must be added to the number of people in your home. Be sure that all the required documentation accompanies this form, including federal tax returns and W2s.
If the US citizen is near the poverty guidelines, it is usually best to have a co-sponsor fill out an I-864.