How many stations does it hold?
It doesn't hold any stations... its just like the web player, it STREAMS only! The music doesn't download to the iPhone/Touch at all, thus no storage issues, but that means you have to be on WiFi or have cell coverage for it work. Kind of bummed about that, but like I said on the other thread I doubt Apple will ever allow ANY app to download music since the iPod is Apple's cash cow.
In my brief testing (using the wife's iPod Touch) the Slacker app worked great, nice job on UI with heart/ban, artist info, etc. Sound quality was as good as the G2 Player too. It would play music thru the USB/dock connector to my Pioneer car radio when tuned to the iPod source, however it defaults to iPod songs so you have to exit out of that every time, go back to the home screen and launch the Slacker App, then it plays your Slacker station.
Also noticed there is a "Buy on iTunes" option that automatically searches the iTunes Store for the album your playing... so its got that little bonus.