I've tried to find out before but can only guess less than 10k of each type. My P6 I bought in December is serial no just over 7000 and my OB6 is about 3400 which I've had about 18 months now.
Thanks
I guess the only trouble with serial number is assuming they start at 0!
My keyboard version (had the module for ages and decided to finally go for keys) which is about 18 days old from the retailer and can't be that old from the factory as it's 'Sequential' not 'DSI' branded is serial number 4217. That's (4217 - 3400) / 18 = 45 units a month! Sort of
Let's work backwards. 4217 / 45 = 93 months. 93 months ago was September 2012. Let's check when it was released. Googles. Earliest reviews I'm seeing are Jan 2016. You'd expect more units to sell in the beginning. So let's revise that. 4217 / (12 x 4.5) = avg. 78 units a month since introduced to the market!
Very rough of course but seems safe to assume then the serial number is simply sequential perhaps with a small offset of a few tens of pre-production units.
So, my conclusion ... not more than 5,000.
As a sidenote, an article on Greatsynthsisers.com suggests between 6,000 and 8,000 of the original Prophet 5's were sold. Interesting that despite the passing years the market size remains comparable.
https://greatsynthesizers.com/en/review/sequential-prophet-5-milestone-and-musical-legend/