You have a couple of options on how to create your expected number of entities.
If you know when the 20 entities are to be generated, then use an Event-Based Sequence block connected to a Time Based Entity Generator which generates entities based on the intergeneration time from port t. In the Event-Based Sequence block, you can specify the intergeneration time and end the vector output with inf to stop entity generation. For example, if you want to generate 20 entities every one second specify your output vector to [repmat(1,1,20) inf];.
If you don't know when trucks will be generated (not deterministic) then you can use an Enabled Gate to stop generated entities from entering your system by closing the gate once you have generated 20. A statistic output from the Entity Generator can give you the count of how many have departed so far.