How to configure that client can use only one or two devices on ONT ?
This command can limit max mac number :
ZXAN(config-if)#max-mac-learn 128 vport 1
What are latest ZTE C300 OLT SW version? We heard about V.2 and V3?
Current (on March 2015) latest ZTE C300 OLT SW version for international market are V1.2.3 or V1.2.5. Another versions You can see in market are either project or country specific.