Geoff Keighley used his Twitter account to reveal that Microsoft will show 20 games at their E3 conference, although he did not confirmed how many Xbox One or 360 games will be.
Keighley obtain this information after talking with Phil Spencer, and went on to say that some Xbox games will be shown on Spike, including exclusive reveals during its Xbox pre- and after-shows. Despite not being confirmed how many titles are destined for Xbox One or Xbox 360, let's not forget that it was Microsoft itself that promised 15 exclusive titles for their new console during it's first year, so it's quite possible a few of them will appear at the event. Keighley also promised a surprise "no one has guessed" for E3.
Microsoft's conference is the first major event of E3 and kicks off at 9:30AM PST June 10.