I dont know of a way to show this per cluster in a list view as the attribute=runtime|maintenanceState is a property and not a metric and as such the count function with the where condition wont work because the input is text *inMaintenance) and tghe SM needs a numeric value
The way i do it is to create a custom group that will have a membership rule for all hosts in maintenance mode. then create a super metric count(${adaptertype=VMWARE, objecttype=HostSystem, attribute=badge|health, depth=1}) This will count the number of hosts in the group (the number of health badges regardless of the health to be exact.
The only problem is that this will show all the hosts in the environment. The only way to have it per cluster is to create a separate group per cluster (create a new Group type and add all the cluster groups under that type) and use the membership rules to limit the members to a cluster. Then when creating your object list use the object type/ "your group type name" and the super metric and it will show you all the clusters and the number of hosts in maintenance mode.
In the below i use it to show the number VMs in a vlan. Each vlan is a custom group that uses the vlan is equal to the vlan id and then i use the count SM to count the health badges of the VMs in the group (will work the same for hosts using the above SM)
The group type
The widget
Widget config with the SM to the right and object type expanded to the left
This is the group type further down the object type list above
I dont know of any other way to show this.