The lower the pilots per aircraft the more days you will have to work, and the more JA and extension you will have to deal with, however movement up is typically quicker.
The higher the pilots per aircraft the reserve length will typically be longer, less flying per month, and movement up is slower.
10-12 is a decent range, below that your QOL will suffer, above that your progression will suffer.
These are all ballpark as things like average stage length, trip quality (credit/day, etc.), and scheduling rules can shift that range up or down.