What does it mean to share a copy of a periodical?

What does it mean to share a copy of a periodical?

The passage below is from The Library: A Fragile History by Andrew Pettegree.

Traditionally the magazine market was littered with optimistic new titles with a short lifespan, but these had staying power. Time was selling half a million copies by 1934, most subscribers, significantly, reading it cover to cover. Esquire (1933), an upmarket journal of men’s fashion, was selling 700,000 copies a week within four years. But none would match the extraordinary circulation of Life, a photojournal launched with prescient timing in 1936. Its superbly framed photographs, chronicling both American life and the evolving European crisis, had attracted 20 million readers by 1938 (sharing 3 million copies), a figure that represented one in every five Americans over the age of ten. Picture Post, launched in the wake of Life in 1938, would play an equally influential role in

shaping self-perceptions of Britain during the war.

I want to ask the meaning of ‘share’ in this context.

It seems to mean among all the copies being subscribed 3 million copies of them are shared.

But that’s just my wild guess and even if my guess is right by chance, I cannot make any sense out of it.

Thanks in advance.