That is a question to answer from the official programme page rather than by assumption. Family coverage and any per-person amounts are terms of the programme and should be verified as published.
Read the family terms from the source, in their own words
Hypothetical example: In other categories the treatment of a spouse and children is set out in statute and regulation, and it varies: an L-2 spouse is employment-authorised incident to status, a TD spouse is not, and EB-5 derivatives receive conditional residence alongside the investor. It cannot be assumed that any of those rules carries over here. The official programme material states its own terms for family members, including whether amounts are charged per person, and that material is where the answer belongs.
Where a household includes a child approaching twenty-one, the age question deserves particular attention because in most immigration contexts turning twenty-one changes a person's eligibility. Read the published terms directly, write down what they say about each member of your household, and if the material is silent on a point, treat the point as unresolved rather than resolved in your favour.