Sophia here from Shopify. Welcome to the Shopify Community, and congratulations on your first sales!
The business information entered when completing the Shopify Payments setup depends on your region as well as the type of business. Typically, the business number is optional for an individual/sole proprietorship. If you've registered your business as a corporation or LLC, for example, you'll likely need to enter the business number.
Let me know where you're operating the business from and I can check out some country-specific resources for more information on registration requirements for your business.