Update on #GTAV Steam owners: 1,146,914 ± 59,680.
It doesn't include non-Steam copies.
More data should be available tomorrow morning.
— Steam Spy (@Steam_Spy) April 14, 2015
I don't know how he is reliable, but it is definitely believable since GTAV is at a constant 200,000 users concurrent or higher. This is just Steam sales and does not count Disk or other clients.
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