Can I assign Microsoft Office 365 Business licenses to more users than the pre-set number?

No, you cannot. When you purchase Office 365 Business, you have to enter the number of available licenses according to your employee headcount. After the purchase, you will create accounts and assign licenses to your employees. Every employee can install Office on a maximum 5 PCs or Mac, 5 tablets and 5 smartphones. If you need a higher demand, you may purchase more licenses for your employees.