Total Nintendo Switch Online memberships are at 9.8 million, excluding free trials.
These subscripts also include family memberships.
So, a little over 1 in every 4 Nintendo Switch owners have a paid online membership.
It doesn't work like that when family memberships are included, because those allow up to eight accounts on one Switch console to play online. A subscription does not equate to a unique Switch owner.
The most interesting information in this PDF is that Switch sell-through in Japan, North America and Europe from January to March 2019 combines for growth of 35% year over year. While I don't have numbers for Japan and the USA at hand, I am pretty sure that those countries' individual growth is below 35%, so Europe drives up the average by showing growth of more than 35%.