According to the USCIS, adoptive parents must have a household income equal to or higher than 125% of the US poverty level for your household size. Your household size includes you, your dependents, any relatives living with you, and the child you wish to adopt.

Poverty Guidelines for the
48 Contiguous States and the District of Columbia

Persons in family/household                       Poverty guideline
 2                                                                $20,300
3                                                                $25,535
4                                                                $30,750
5                                                                $35,975
6                                                                $41,200
 7                                                                $46,425
8                                                                $51,650
For families/households with more than 8 persons, add $5,225 for each additional person.