What is the optimum value for omp_set_max_active_levels in OpenMP?

I have a specific example, but I'm also interested in a more general answer. Given the following code, and an evenly balanced binary tree:

void mainTraverse(tree *node) {
    omp_set_nested(1);
    omp_set_max_active_levels(4);
    recurseTraverse(node);
}

void recurseTraverse(tree *node) 
{
    if (node == NULL)
        return;

    #pragma omp parallel
    {
        #pragma omp sections
        {
            #pragma omp section
            recurseTraverse(node->left);

            #pragma omp section
            recurseTraverse(node->right);
        }
    }

    // lengthy per-node computation here
}

According to the OpenMP specification:

The omp_set_max_active_levels routine limits the number of nested active parallel regions.

But what does this mean exactly? Is 4 the optimum value for set_max_active_levels? How many threads will this code create?