There are no limitations on number of concurrent calls that you can make using Zentrunk. Zentrunk servers are elastic in nature. These servers can scale up or down depending on the traffic. No additional configuration is required to add more capacity. And since Zentrunk SIP Trunks are charged on a pay-as-you-go basis, you are only charged for the minutes you use.
There is a limitation on Calls Per Second (CPS). CPS indicates the number of calls that can be initiated every second. By default each account can initiate 1 Call Per Second. Please contact Plivo Sales team if you would like higher CPS.