Question: I am planning to use an USB hub and plug in 6 USB ethernet adapters to use my laptop as 6 different users, with 6 different IP addresses to do some network testing. 

What are the possible issues?

Answer: 
If you could tell us what kind of testing you're trying to do, it might help us determine whether this particular setup will be the most beneficial.

I don't think this will cause any issues other than limiting the available bandwidth for each Ethernet adapter (although, as Kelbizzle suggested, a single USB port may not be able to power 6 Ethernet adapters, so you may need a powered hub). If you're trying to maximize the load on your network, you'll be limited by the bandwidth of the USB bus--480Mbps for USB2, and 12Mbps for USB 1.1 (also called USB full speed). Note that it will only take five 100Mbps Ethernet to saturate a USB2 bus if you're running a load test.

In contrast, you would have more than twice the bandwidth available if you use one or more (non-USB) gigabit Ethernet adapters. You can simulate multiple machines using VMware, VirtualBox, VirtualPC, etc. If you use bridged networking, each VM will get its own IP address on the LAN.

If you're trying to test whether your software can handle a heavy network load, it's also possible to create a completely virtual LAN, eliminating the need for any physical network interfaces (along with any physical bottlenecks associated with the physical network interfaces).

If, on the other hand, you're trying to test whether your network itself can handle heavy traffic, you may need to use several physical networked machines to sufficiently saturate the network.
One possible outcome is that it messes up your wi-fi connection or any existing connection to the internet. Just an FYI. Other than that, good luck!