1.4.25.How many concurrent queries can Virtuoso handle?

There is no set limit. As with any DBMS, response times get longer if there is severe congestion.

For example, having 2 or 3 concurrent queries per core is a good performance point which will keep all parts of the system busy. Having more than this is possible but will not increase overall throughput.

With a cluster, each server has both HTTP and SQL listeners, so clients can be evenly spread across all nodes. In a heavy traffic Web application, it is best to have a load balancer in front of the HTTP endpoints to divide the connections among the servers and to keep some cap on the number of concurrently running requests, enforcing a maximum request-rate per client IP address, etc.