Is it possible to count all the positive colonies in a well for the virus titering protocol and what magnification of the microscope should I use?
You do want to count the number of positive colonies in the entire well of the 24-well plate. There needs to be at least 50 positive colonies to be accurate but if there are too many to count (more than ~125) in a particular well you would need to step up to the next highest dilution well and count those.
You won't be able to look at the entire well of the 24-well plate at one time. You must set a method by which you scan back and forth and up and down across the entire well. Starting at the left with the top of the well in the field of view scan right. Then move down until the bottom of the field of view falls just outside the top of the field of view and proceed back left and so on. This is usually done with the microscope set at 20X.