Under 6.7 it is 64 hosts per cluster, which I have ran slightly higher than that (~75) before splitting the cluster in two. With the different DRS capabilities and the tie-in with vROps, VMs can move between clusters without any issue. From a management standpoint, vDS and Datastores/Datastore Clusters are a Datacenter level object, not a cluster level, so having multiple clusters in the same datacenter means you have have the same vDS and Datastores/Datastore Clusters across all the hosts for ease of management.
The link previously provided (https://configmax.vmware.com/guest?vmwareproduct=vSphere&release=vSphere%206.7&categories=2-0) is always the best source for these since they can change even with updates. Also, you must consider other maxims, VMs per host, Datastores per host, vDS ports per host, etc. It's often easier to create a table and lay out your expectations that way, I like to use a white board personally.
Overall, having multiple clusters in a datacenter, maybe doing the same functions and maybe not, are totally a design decision and has no impact on operations. Now one thing to think about though, is if you wish to maintain N+1 on hardware, this will be at the cluster level, not datacenter level, so each cluster has it's own N+1.
