The number of classes you need depends on what goods and services you sell under your brand. There are 45 different standardized classes you have to choose from. Classes 1-34 cover goods, and classes 35-45 cover services.
If you sell a wider variety of products and services, you can choose multiple classes. The final price will then depend on the number of classes you'll include in your application.
For example, let's say you offer a combination of goods and services via your website - education and training services, stationary, downloadable educational materials and mugs. The most fitting classes for you would be 41, 16 and 21. Class 41 includes education and training service, class 16 stationery and class 21 mugs.