The number and exact choice of classes for your trademark application will mostly depend on what goods/services the brand that is to be protected offers. A clothing company might register their trademark in a single class - Class 25, which includes clothing. On the other hand, an online platform offering meal delivery might want to register its trademark in three classes - Class 35 for e-commerce, Class 39 for transportation and Class 43 for services providing food and drinks. You can find the overview of the 45 classes here.
On average, owners of registered trademarks tend to register 2-3 classes; however, numerous well-diversified brands have registered their trademarks in over 10 classes.
The choice also depends on the existence of similar already registered trademarks within the same class. If such trademarks exist, their owners can oppose the new application and prevent it from being registered. Applicants sometimes choose to limit the number of classes on their application to avoid such opposition; other times, it's done after the opposition has been raised. You can find similar registered trademarks for a given class by searching the IP office's public database or using a professional trademark search service (at Trama, we perform trademark search free of charge).
If you are unsure how to select appropriate classes for your business, a trademark attorney can help you draft your application. At Trama, we offer a free lawyer's check where we evaluate the registrability of your mark and also provide a recommended list of classes.