Update instance count is failing on prod rack

Currently in our prod rack we do have 5 instances. we need to change minimum instance count to 6. please tell us the correct procedure

We tried using below command
convox rack params set InstanceCount=5

somehow it increased instance count to 6 but after sometime it removed that extra instance.

what can we do ?

Not sure if that’s a typo in what you’ve said you tried but it should be:

convox rack params set InstanceCount=6

Do you also have the Autoscale parameter set to true ?