How many companies allow students from any branch in campus placement?
These depends on the college and its tie up's. Although in IIT's NIT's and most state government college of good repute, are visited by the companies which offers a free placement cell to all branches to sit for there interview and selection rounds. So if you are in a good college then you will get quite good number of companies available to open placement sessions. Around 60% of the companies recruit from all branches. So don't worry. For more specific answer to your college, repost the question with the name of the college on the career360 qna section so that we can provide you the required info.
I hope this answer helps. All the very best for your future endeavors!