I have a figure of 17k in my head, not sure if that's correct without looking it up. I think the other side of the 2014 mule pair, the Britannia obverse coupled with the lunar horse reverse is supposed to be more common but in my experience, you don't see so many for sale.
PR70 Proof 2017 Sovereign going for around 530 pounds with around 2 days left
There was a mule 2014 Britannia, the Queens side had a plain edge by mistake, can't remember how many there were suppose to be.
Looking at Ebay they seem to sell for at least £40, so that's not a bad premium if you got them around spot at the time
But this is a Royal Mint coin from an established series so very different to a random mint with random coin.