Hello Steve. In a production grade ACI fabric, there would always be 3 APICs (you can add more for scale but let's put that to one side). APICs are physical appliances that handle the management plane of the fabric (i.e they are not dependent on the data or control planes).
That's a real fabric. The simulator is, as its name suggests, a virtual representation of a very small fabric. Because it simulates one spine and 2 leafs plus the APIC, we needed to keep it limited. Simulating all that in one OVA starts to consume a lot of resources. For that reason, you will only have one APIC when using the simulator.
As for the 'primary vlan', I looked at the topology diagram on dCloud and to me it just looks like a VLAN they set up outside of ACI to connect all the other things via OOB network. This is specific to the dCloud lab and nothing really to do with ACI.
