How many scissors you need depends on the situation. If your scissors are dull and can't be used for a week, you can use a pair or a set of trimming scissors. But imagine that you drop the scissors when you are halfway through trimming your dog's hair: the scissors will not work properly and the dog's hair will only be half trimmed. For professional use, you need a pair of each type of scissors in case they need to be sharpened.
As a beginner, you should first buy a pair of straight scissors and thinning scissors. With these, you can trim everything, and they are cheap. Professionals use scissors of different qualities and use different scissors according to the type, style and size of the dog.