Hi @packet2020,
Thanks for raising your questions in Community. You are right. If you want to expand 200 leaf switches you should add 4th APIC into cluster. And we recommend that you have at least 3 active Cisco APICs in a cluster, along with additional standby Cisco APICs. In most cases, we recommend a cluster size of 3, 5, or 7 Cisco APICs. We recommend 4 Cisco APICs for a two site multi-pod fabric that has between 80 to 200 leaf switches, fit your scenarios so far.
And you'd better take below actions before you execute, prior to initiating a change to the cluster, always verify its health. When performing planned changes to the cluster, all controllers in the cluster should be healthy. If one or more of the Cisco APICs' health status in the cluster is not "fully fit," remedy that situation before proceeding. Also, assure that cluster controllers added to the Cisco APIC are running the same version of firmware as the other controllers in the Cisco APIC cluster. More details you can refer to Cluster Management Guidelines.
https://www.cisco.com/c/en/us/td/docs/dcn/aci/apic/5x/getting-started/cisco-apic-getting-started-guide-52x/cisco-apic-cluster-management-52x.html#concept_32638F2B2E4648C5AD1D32485F7C9914
Please follow Expanding the APIC Cluster Size guidelines and steps in Expanding the APIC Cluster Using the GUI.
Step 1
On the menu bar, choose System > Controllers.
Step 2
In the Navigation pane, expand Controllers > apic_name > Cluster as Seen by Node.
For apic_name, you must choose a Cisco APIC that is within the cluster that you wish to expand.
The Cluster as Seen by Node window appears in the Work pane with the APIC Cluster and Standby APIC tabs. In the APIC Cluster tab, the controller details appear. This includes the current cluster target and current sizes, the administrative, operational, and health states of each controller in the cluster.
Step 3
Verify that the health state of the cluster is Fully Fit before you proceed with contracting the cluster.
Step 4
In the Work pane, click Actions > Change Cluster Size.
Step 5
In the Change Cluster Size dialog box, in the Target Cluster Administrative Size field, choose the target number to which you want to expand the cluster. Click Submit.
Note
You cannot have a cluster size of two Cisco APICs. You can have a cluster of one, three, or more Cisco APICs.
Step 6
In the Confirmation dialog box, click Yes.
In the Work pane, under Properties, the Target Size field must display your target cluster size.
Step 7
Physically connect all the Cisco APICs that are being added to the cluster.
In the Work pane, in the Cluster > Controllers area, the Cisco APICs are added one by one and displayed in the sequential order starting with N + 1 and continuing until the target cluster size is achieved.
Step 8
Verify that the Cisco APICs are in operational state, and the health state of each controller is Fully Fit.
Before your take actions, you must first set up any Cisco APIC that you will add to the cluster. For information about setting up a Cisco APIC, see Setting up the Cisco APIC.
Lastly, additional you could learn more about APIC Cluster Deployment Considerations parts in Cisco Live: Cisco ACI Multi-Pod Design and Deployment.
https://www.ciscolive.com/c/dam/r/ciscolive/global-event/docs/2023/pdf/BRKDCN-2949.pdf
