2 children total can be adopted, they are at honorhall orphanage, but the newly added ones are a girl in green in whiterun, boy in blue at solitude's stables, boy in blue in dawnstar, usually near the mine and a girl in blue in windhelm selling flowers, with the children there are also some you can make parentless to adopt.
For exampe if you kill the father of the twins in rorikstead, they are sent to the hall and can be adopted