Hi Friend,
As per configuration maximum, 10 is the max vCenters you can add into linked mode group. As it is specified by VMware, it is mean that it is tested and recommended by VMware. If we go beyond that we can not know whether it will work or not . Hence it is better NOT to take risk (Lot of efforts requires to set up 10 VC itself).
As per me, rarely anybody will go for 10 itself.
Also as per the linked mode requirements/prerequisites: all vCenter Servers in a Linked Mode group are registered to the same vCenter Single Sign On server.
Refer requirements/prerequisites in details::: VMware vSphere 5.1
In order to avoid single point of failure @SSO, you can implement SSO HA (using third party load balancer). You will get KBs/blogs to configure the same @google.
Please also explore "Multisite SSO deployment"
VMware KB: Identifying VMware vCenter Single Sign-On server deployment mode