19 states allow it
15 ban it completely
the rest have exceptions for location and family members
Because it was more complex than just that single number (the 15 that completely forbid it) I included the link so you can verify for yourself that only 15 states have an outright ban.
What is considered a 'private setting' and a family situation is not for me to decide, we're talking about consumption here, not about sales.
So in 35 states drinking by minors in some settings is legal, and in 19 of them there are no restrictions.
By the same logic, in 31 states there are some restrictions on alcohol consumption, and it is completely forbidden in 15 of those.