Minimum servers in calla manager cluster should contain

[Yenosh]

How many minimum  servers should available in CUCM cluster

[Dennis Mink]

As always it depends. You need a minimum of 1. And 2 to make it redundant. Then you might want to add more depending on geographical spread. How big is your deployment?

[Jaime Valencia]

I suggest you review the SRND on this topic.