When to use different number of instances in two different availability zones?

Hi.There are cases like,

1.The company owns RI tied to one specific AZ and still has room to spare, so we want to set up as many instances as possible in that AZ, but capacity is not a problem if the AZ with more instances fails.

2.New instance type with better performance has shortage in one specific AZ, so there are difference about number of instances bw AZs due to instance type's performance efficiency.