"Does this mean there is 18k on the Bulls? Is there any way to tell if this is from a single bettor?"
No and NO
It means that there is $12000 that you can bet on Cleveland-2400.These offers were placed by someone or a bunch of bettors that want you to accept it because there money is on chicago.
In this case i think it was from a single player,but no real way to tell.
The other $617 is an offer to take the Bulls at +1800
The closer to gametime the closer the gap of the line will get.
EX:It might be -2000-+1900 by game time
I hope i explained ok
hno:I'm at work and sorta rushing this post