"I don't make forecasts," said Ballmer, in an interview published Monday in The Wall Street Journal.
"It's partly how many we can get made, it's partly how much we can—can not only build a great product, but how does the word of mouth work, how effective is the advertising we'll do?" said Ballmer.
Ballmer also admitted that Microsoft "missed a cycle" in phone development due to R&D and execution problems. Ballmer, however, told the newspaper he's confident Microsoft can make a comeback in the phone market. "Job one here will be selling a lot of phones, and if we sell a lot of phones, good things are going to happen," said Ballmer.
Microsoft's credibility in phones took a severe hit when it pulled its KIN line from retailers earlier this year amid dismal sales after just several weeks on the market.
The debacle led Microsoft's board to slash Ballmer's bonus for the past fiscal year in half, according to financial documents the company filed last week with the Securities and Exchange Commission.
Microsoft has confirmed it will formally launch Windows Phone 7 on Oct. 11, at an event in New York City.
Windows Phone 7 devices, from a list of Microsoft partners that includes HP, Samsung, LG, Toshiba, and Sony Ericcson, are expect to go on sale about a month later. AT&T and T-Mobile will reportedly be the U.S. network carriers.
Many observers believe Windows Phone 7 is Microsoft's last chance to become a significant player in the increasingly important mobile market. Current products, based on the Windows Mobile operating system, have proven to be also-rans against more competitive offerings from Apple, RIM, and newcomer Google, with its Android-powered devices.
RIM, with its Blackberry OS, holds the largest U.S. market share for mobile platforms, with a 39.3% stake as of July 30, according to market watcher Comscore. Apple's iPhone is second, with a 23.8% share, followed by Google's 17% share. Microsoft was fourth, with an 11.8% stake.