Hi,
There is no such documentation that counts the max number of switches in ERPS ring .
Max number of the switches that can be connected to the ERPS ring depends upon the design of the network.
Refered this question to experts and they refer to some topologies they used 10 switches on a ERP ring . So, it only depends on your network design (e.g. number of clients, routes ..)
If you still haven't try to connect the 8 switches , I suggest you go for it and try .
HTH