"Hello friends! Cuphead has only been out for two weeks and we never could have dreamed of the reception we’ve received from our amazing fans! Today, we wanted to take a moment to thank each and every one of you for your support and we’re excited to share that Cuphead has gone platinum with over 1 million units sold!"
http://studiomdhr.com/cuphead-goes-platinum/
Steamspy has the PC version around 576k so the Xbox/W10 and GoG version should be around 424k.
http://steamspy.com/app/268910