There are usually a lot of companies participate in major career fairs. Such as the Engineering/Science fair, they will have a lot of companies that are specializing in different industries; such as energy, utilities, computers, construction, medical and so on. In other words, your set of skills might not be compatible for all of the companies setting up booths in the fair. As such, you should focus on the companies that can utilize your professional knowledge and skills.
All fairs should have a list of participants either on their advertisement or on their website. It is crucial for you to analyze the list and pick out the companies that you are interested in the most. Some career fairs such as the ones held by universities and colleges would even split the categories into majors and faculty for students to pinpoint their targets more easily (And did I mention that the college career fairs are one of the best fairs to attend for jobs regardless of your age and experience?).
The number of companies that you want to pick out is determined by how much homework that you would want to do, as I would recommend you to research thoroughly into each company to prepare for the conversation with the recruiters. Make sure to choose the companies that you are most interested or you have the most experience on as we would want every bit of your effort count. Normally, pick out 10-20 companies that you would want to work for within the participant list.