Spot admission is basically all about the seats left after 1st & 2 ND rounds of Counseling and including upgradation.

No you can't go in all the colleges as it is obviously tough to do that
Basically you are called by your rank and a/c to the number of seats left in the college you can get seat.

Hi Lydia, 

Spot Admission is basically using score of the exam and going to colleges where they allow the same, this would have a certain number of seats which would be only allotted for the same

All the best 

I hope this helps you