You can use either FileMaker Server 12 or FileMaker Server 12 Advanced to do Custom Web Publishing. See here for additional information:
You'll notice that you get additional users (100 versus 200) when you go to FileMaker Server 12 Advanced.
Thanks for your response, I had just been looking at that URL. It does appear that the only difference is the 100 versus 200 concurrent CWP users. I'll have to try Server 12, to see how my application responds.
The limit is on the number of concurrent connections. If you have more than 100 people hitting your CWP web site simultaneously you’ll need advanced.