Server is required. Concurrent connections are sold in 5 connection packages.
In order for clients to connect to solutions hosted on FileMaker Server 13 via FileMaker Go or WebDirect, a concurrent connection must be available on the server. FileMaker Server 13 includes support for one concurrent connection, and you can purchase additional connections in groups of 5, up to a total of 50 connections. (You can also purchase Server in such a way that the concurrent connections aren't restricted.) To get a sense of the pricing for these concurrent connections, go here https://store.filemaker.com/US/ENG/LIC/ and adjust the quantity for FileMaker Server 13.