I guess it depends on the definition: I'd say owning Bitcoin means owning owning your keys. Considering there are only 53,218,057 funded Bitcoin addresses (based on yesterday's data), and considering many Bitcoin owners have more than one funded address, there are far less than 50 million Bitcoin owners worldwide.
On the other hand, if I use a broker to buy shares of a certain company, I can say I own shares of that company. So I guess (by now) it's okay to say you own Bitcoin when you "have" some on an exchange or ETF. By that definition, 1 in 6 or 7 sounds plausible.
There is some nuanced perspectives about ownership. The conclusion I've come to is that if you own it if you can keep it. This is possible for Bitcoin like nothing else.
See also the "If buying isn't owning, piracy isn't stealing" discussion.