comparing to Cisco IOS , we can create any number of loopbacks on router , but on Junos you can create only one loopback interface (even logical units from same loopback) in the routing instance , did any one suggest or think about the reason for this design ? and did Junos has another logical interface can be used in the place of loopback ?
You can add an individual lo0.x to each routing-instance present. So if you configure multiple routing-instances, you can configure multiple, separate loopback interfaces.
And this is 'the proper way to do it', instead of multiple loopbacks per single routing-instance. The purpose of the loopback interface is to act as both an anchor interface for identification, but also an 'always-up' anchor interface. There should be no need for multiple anchors or identification points for a single instance of a network-computing entity (my ad hoc alternative term for a routing-instance). As others have stated here already, it's not just plausible but advisable to merely add more addresses to the single loopback interface instead of creating more interfaces.
Not being able to have that kind of functionality in a routing-instance would be disastrous for things like OSPF and others, so it's possible to place as many as necessary into each routing-instance you decide to configure (one being as many as necessary).