How about 1 envoy per era?
In a similar thread, folks were saying even 3 was not enough because the ai would push out many more... so you could liberate a cs than immediately be at war with it. Apparently even 9 or 11 envoys sent by the ai was not unheard of.
Personally, I have no problem with that. But if you think a liberator should be an automatic suzy, its something to consider.