The number of business units that NTO needs to purchase depends on several factors, such as the number of brands or geographies they operate, the need for data or content segregation between them, and the number of Marketing Cloud users they have or plan to have. These factors affect the complexity, scalability, and cost of their account hierarchy and governance. The number of Sales Cloud sandboxes or testing/development business units are not directly related to the number of business units they need to purchase, but rather to their testing and deployment strategy. References:
https://help.salesforce.com/articleView?id=sf.mc_overview_business_units.htm&type=5https://help.salesforce.c