More than 2,000 in fact, encompassing Rock Band Network, LEGO Rock Band, Rock Band 1, 2 and soon, Rock Band 3. In an effort to help make finding out if the song you want is among those, the developer has created an embeddable web widget that lets you search by artist, song title, year and genre (then sort within those results).
It's really pretty handy -- we've embedded it after the break, actually, so you can give it a try. If you'd like to stick it on your websitethingy, grab the widget here. Those familiar with iframes and other such web two-point-oh terms will know what to do with it.