I know that until EF6 is recommended that break project in small models/contexts, I remember that I saw something around 10 classes per context. Well, this still applicable for EF7 / Entity Framework Core ?
The EF Core does not limit the number of entity types that can be added to a model. Models with high numbers of entity types only affects the performance of the initial model creation, but this model is cached so creation is only performed once per context type.
What has not changed from EF6, however, is that using LINQ queries on highly-connected graphs of many related entity types can result in slow, complex, or buggy behavior.
More info: checkout https://docs.efproject.net/en/latest/efcore-vs-ef6/ for more information on comparing EF Core and EF 6, and https://docs.efproject.net/en/latest/modeling/ for more information on modeling your database.