Direct answer
The USPTO uses 45 international classes to organize goods and services. Your application should include the classes that match the goods or services connected with the mark and your filing basis. The base application fee is charged per class.
There are 45 international classes
Classes 1 through 34 cover goods and Classes 35 through 45 cover services. A single business can legitimately need more than one class when it sells different categories of goods or provides different services.
More classes increase government filing cost
For most Sections 1 and 44 applications, the current base fee is $350 per class. Two classes therefore start with $700 in base USPTO application fees before any additional fees or professional services.
Do not add classes just to make protection look broader
Each class needs a valid legal basis and appropriate goods or services. Overbroad filing can increase cost and complexity without creating useful rights.
Use the actual commercial offering to choose classes
Describe what the business sells or provides, not only the industry label. The classification decision is tied to the specific goods and services in the application.