When the num_instance() API will be called?

I have written a model including a read only container and list. And I implement the model using callpoint. I have implemented the get_elem(), get_next(), and num_instance() APIs for the callback. But the num_instance() is never called when I query this read only container and list.
Could you let me know when the num_instance() API will be called? Thanks.

My model is as below.

module test-ro {
    namespace "http://test";
    prefix tro;

    container test-ro { 
        config false;
        tailf:callpoint test;    
        list test-group {
                key test-group-id;
                description "Represents a group of test.";

                leaf test-group-id {
                        type int8;
                        description "Identity of the test-group entry.";
                leaf generation {
                        type int8;

Try for example:
$ confd_cmd -d -d -c 'maapi_num_instances /test-ro/test-group'

@cohult, I meet this issue now. What you say does work and I see the “num_instance” is called. But, I use the external database and don’t use confd directly. I register callback and “num_instance” ,“get_element”,“get_next” in my external database.
the yang is just like @yi_z_zhao yang.
Can you give me some instruction? thanks.