"We may sell a few million," Ballmer said during Monday's Worldwide Partner Conference in Toronto.
He cited forecasts that 375 million new Windows PCs will be sold in the next year, PC Magazine reported.
While Ballmer said "I don't know how many of the 375 million" will be Surface sales, he promised the Surface tablet "will have a distinct place in what's a broad Windows ecosystem."
Surface for Windows RT will be available in a 32GB and 64GB model, priced "comparable" to other tablets, Microsoft said during its June 18 announcement of the tablet.
After the Surface announcement, there had been speculation Microsoft's hardware partners might feel betrayed by the notion of their software partner producing a device that would compete their own tablet offerings.
Balmer tried to allay those fears.
"The importance of the thousands of partners that we have that design and produce Windows computers will not diminish," Ballmer said. "We have a mutual goal with our OEM partners to bring a diversity of solutions, Windows PCs, phones, tablets, servers, to market. And what we seek to have is a spectrum of stunning devices, stunning Windows devices."