vRealize Log Insight 2.5 improves on the clustering in previous versions with an Integrated Load Balancer (ILB) which allows you to distribute load across your cluster of Log Insight instances without actually needing an external load balancer. The advantage of this over an external load balancer is that the source IP is maintained which allows for easier analysis.
The minimum number of nodes in a cluster is three, the first node becomes the Master node and the other two become Worker nodes. The maximum number of nodes supported is six, though acording to Mr Log Insight himself, Steve Flanders, the hard limit is more:
@sammcgeown yes though hard limit in product is much higher. How big do you need?
— Steve Flanders (@smflanders) February 11, 2015
The Log Insight appliance comes in four sizes: an Extra Small, for use in labs or PoC, through to Large, which can consume a whopping 112.5GB of logs per day. Those figures scale in a linear fashion for clusters, so a 3-node cluster of large instances can consume 337.5GB per day, from 2,250 syslog connections at a rate of 22,500 events a second. See more on sizing vRealize Log Insight.