parallel_max_servers and the number of sessions involved in a SQL

I'm trying to understand how Oracle defines the limit on the number of sessions involved in a SQL statement (a select on big table using a parallel hint)
I ran some tests and I found the following:  

                               number_of_sessions = 

where
cpu (host) n_max

1              40
2              80
3             120
4             160
5              200
6              228
7              228
8              228
 

for my test I used 12.2.0.1 installed on a virtual box (the host machine has 12 cpus)
can someone explain the n_max/cpu relationship?  

Best regards
Ahmed