Hello, 1 Web/SSO server will work and you can point multiple vcenters to it to get a "single-pane-of-view" - just remember this is only for the web client Only. Akopylov is spot on with his comments, and without repeating anything, you will find this link useful When to Centralize vCenter Single Sign-On Server 5.5 | VMware vSphere Blog - VMware Blogs
And these two sessions from VMworld 2013