"We may sell a few million," said Ballmer, during a keynote presentation at Microsoft's Worldwide Partner Conference in Toronto.
Microsoft shook up the hardware industry, and may have ruffled some hardware partners, with its surprise unveiling of Surface last month. Surface marks Microsoft's first move into the PC manufacturing business, an area it has traditionally left to partners like Hewlett-Packard, Dell, and other so-called OEMs.
At WPC, Ballmer appeared anxious to quell partners' fears that the company plans to be a major player in computer hardware. He said that the point of Surface is mostly to create a showcase for the full potential of Windows 8 on tablets.
"We had to make sure that we had a piece of hardware that was unique in its ability to be that leading-edge hardware that really showed off Windows 8," said Ballmer. "But Surface is just a design point; it will have a distinct place in what's a broad ecosystem. And the importance of the thousands of partners that we have that design and produce Windows computers will not diminish."
[ Moving from Windows 7 to Windows 8? Here's what you need to know now: Windows 8 Upgrade FAQ: How To Go Metro. ]
As a result, Surface sales will represent only a small portion of the 375 million Windows PCs and tablets expected to be sold worldwide in the next 12 months. "We may sell a few million, I don't know how many, of the 375 million, but we need partners to have that diversity of devices," said Ballmer.
To reassure buyers and hardware makers that there will be many options beyond Surface when Windows 8 ships in October, Tami Reller, CFO for Microsoft's Windows group, showed off a number of new devices at WPC on Monday, including tablets and hybrid systems from Asus, Acer, HP, and Lenovo.
Microsoft has good reason to court hardware makers more strongly than ever. If they sense the company is trying to move too aggressively into their turf, some could defect to the Google camp and produce systems that run Android or Chrome.
On Monday, Microsoft announced that Windows 8 would be released to manufacturing (RTM) in August and that systems running the OS would be available in stores by October. With Windows 8, Microsoft has essentially forked its operating system. One version will run on standard Intel and AMD chips, while Windows 8 RT will run exclusively on ARM-based tablets and is only compatible with apps downloaded from Microsoft's online application store.
Windows 7, Vista, and XP users will be able to upgrade to Windows 8 Pro for $39.99, and consumers who purchase a new Windows 7 PC between now and Jan. 31 can upgrade for just $14.99.
At this year's InformationWeek 500 Conference, C-level execs will gather to discuss how they're rewriting the old IT rulebook and accelerating business execution. At the St. Regis Monarch Beach, Dana Point, Calif., Sept. 9-11.