We have different saas applications to deploy for our clients with 2 different configurations: some applications are exclusive to some clients (one or multiple apps for a single client) while others are shared between clients (same app for all clients but with different accounts). What's the best strategy (in terms of HA, security, etc.) to deploy them: using a big single mutual Kubernetes cluster for all of our clients or individual kubernetes cluster for each different client?
I'd say this is somewhat opinion based, but given you have resource limits, network policies, namespaces and RBAC you can use a single cluster for multiple clients.
You just have to make sure your cluster is resilient to failures if you have a single cluster for everything. For example, in AKS you can use availability zones