ソースコード

using System.Linq;
using UnityEngine;

public class Example : MonoBehaviour
{
    private void Awake()
    {
        var meshes = 
             GetComponentsInChildren<MeshFilter>( true )
            .Select( c => c.sharedMesh )
        ;

        var skinnedMeshes = 
             GetComponentsInChildren<SkinnedMeshRenderer>( true )
            .Select( c => c.sharedMesh )
        ;

        var count = meshes
            .Concat( skinnedMeshes )
            .Sum( c => c.triangles.Length / 3 )
        ;

        Debug.Log( count );
    }
}

参考サイト様