Yours, Mine and Ours (1968) on Netflix

With her children, widowed nurse Helen North moves to San Francisco to work at a naval base. There, she meets Naval Officer Frank Beardsley, himself widowed. They are immediately attracted to each other and go on a date. Although they feel their attraction to each other could lead to a more permanent relationship, they decide to call it off when they learn that she has eight children and he has ten. However, Frank's colleague, Naval Officer Darrell Harrison, who also knows Helen, sees the attraction between the two and does whatever he can to push the two together. Despite the obstacles of such a large combined family, and their respective children's dislike of the idea of having a new parent, Helen and Frank decide to get married. They end up having all eighteen of their children living under one roof with them. Together, the children have a "ours" and "theirs" mentality toward their new family unit. The only real bond that immediately emerges is between Helen's adolescent son Philip, who wants so much to be known as a "Beardsley", and Frank's oldest son Mike, who is Philip's protector in navigating through the trials and tribulations of the lives of sixteen other siblings. But things change even more in the household when Mike makes a decision about his future, Frank and Helen make a decision about the children, and when it looks as if there will be an addition to the family, who will not have the burden of being known by any of the other children as one of "them".