Greetings!
Please read this document first - https://www.vmware.com/files/pdf/products/nsx/vmw-nsx-network-virtualization-design-guide.pdf - It really is an excellent resource. Additionally, I would also recommend checking out the VCA-NV self paced curriculum on MyLearn. Last time I checked you could take it free of charge, and it will provide some good 100-level content. I would highly suggest checking out the above before trying to test anything out. That's just my two pennies though....
If you are purely talking test / kick the tires - it depends
1. If you are looking to strictly test Distributed Firewall functionality, you can do that on one host. You basically just deploy and register the NSX Manager - then prep the target host. From there create the rules and test away.
2. If you are talking about Distributed Logical Routing (DLR) and other elements, you could technically get away with two hosts but three is generally the best. The controllers deploy in a set of 3, with the intention that each controller drop on a dedicated host.
3. The # of network ports you require per server varies on your setup, however at a minimum you can simply define a single network interface for a VXLAN transport zone. To the physical world, it is simply present as a VLAN that all hosts contribute a NIC to (in this example).
4. You do not require a layer 3 switch to test it out. The premise of DLR is to abstract VM routing and switching into software, and remove that burden from the physical layer. Could / Would there be situations where a L3 switch would sit in that topology? - Yes - is it required for you just to test? - No.
Hope it helps!
