Ok I did the math, you can get 909 stars.
The menu says you can only get 808 stars, because it says the max you can get per gig is 8, but you can actually get 9 if you get 6 stars for "gold starring" the song and 3 more for getting Diamond on the challenge.
So if you do the fan trick (play beginner vocals and put the mic up to a fan or air conditioner to get 100% on the song) you will get
808/8= 101 x 6 = 606
and the challenges (which you can NOT do on beginner difficulty) you can get
101x3 = 303
which equals to a maximum of 909 stars.
which if my math is correct....
means you need to average a platinum star on each song challenge for the 808 star achievement if you do the fan trick...... no small feat!!
Please feel free to correct me if I am wrong.