One web application or multiple web applications when using multiple databases?

I'm developing an ASP.NET MVC 4 web application hosted on IIS which will be using multiple databases on a single SQL Server 2014 instance (each database for a different company). The web application will be used for analysing and summing large amounts of business data.

What is the best practice in this kind of situation? Will there be performance issues if only one web application and pool is used with multiple databases? One database may have 1-20 users.

Which one would be a better choice, A or B?

A) 1 web application and 10 databases

B) 10 web applications and 10 databases