Hi I have a form in which People Picker is kept having multiple input

You can use the MNI feature of Appian.
forum.appian.com/.../Multiple_Node_Instances

Can this kind of feature can be implemented to initiate a new process (using send message or sub process) too multiple times with different assignee each time??

Hello Aayusha, we can use MNI on a subprocess and have the sub process use any components/smart services it wants to use.
Try MNI on your subprocess node in the parent process, spawn new for as many users you have picked using your People Picker, and then in the assignment pass in this array of picked people[tp!instanceindex].

I am not getting the use case where we would want to MNI a Send Message. MNI is configured using the "Other Tab", which is not available on events (example - Send Message), however we can have a Send Message Event in a sub process and then MNI on a sub process.
Hope this helps.