Hi Hanu,
If we create any custom service with create operation it will allow to create only 1 instance at a time.
if you want to create multiple instance in single request follow the below steps:
1. Create another Custom BO, and declare auto generated ID in root level and in the Item node define all your first BO structure. Item node must be in 0,n association.
2. Now create a custom web service with create operation on this custom Bo(second BO), by selecting only Item level elements(first BO elements).
Now you can create multiple instances in your custom BO (first BO).
Regards,
Shiva G