TextMesh Pro TextInfo not initialized on 'Start()'

You need to run ForceMeshUpdate method before reading textinfo @Mese

  m_textbox.ForceMeshUpdate();
  Debug.Log($" {m_textbox.textInfo.characterCount}");

TL;DR; Try calling textMeshPro.ForceMeshUpdate(), before accessing the TextInfo or the likes.

I'm not sure, if that's still around in the latest version, but TextMeshPro Components used to have a ForceMeshUpdate() method. We ran into issues that text meshes were updated too late in the frame, and with this method we could force the availability of 'up to date' mesh datat to be used, when needed.

That won't work if "Awake" hasn't been called on the Text component yet. This solution works always, by activating every parent in the tree up to the root, then restoring the active state of each.

 public static int GetLineCount(this TMP_Text label)
 {
     var disableds = label.transform.EnumerateAllParents(true).Where(p => !p.gameObject.activeSelf).ToList();
     disableds.ForEach(p => p.gameObject.SetActive(true));
     label.ForceMeshUpdate(true);
     if (label.textInfo == null)
     {
         Console.Log($"Label {label.name.Quoted()} doesn't have a 'textInfo' object. This is usually because it has not been 'Awoken' yet. activeSelf: {label.gameObject.activeSelf} activeInHierarchy: {label.gameObject.activeInHierarchy}");
         // Try to salvage:
         return label.text.Empty() ? 0 : (label.text.Count('\n') + 1);
     }
     disableds.ForEach(p => p.gameObject.SetActive(false));
     return label.textInfo.lineCount;
 }

Utility:

         public static IEnumerable EnumerateAllParents(this Transform go, bool includeSelf = false)
         {
             if (!includeSelf)
             {
                 go = go?.parent;
             }
             while (go)
             {
                 yield return go;
                 go = go.parent;
             }
         }
         public static string Quoted(this string s) => s == null ? "null" : $"\"{s}\"";
         public static int Count<T>(this IEnumerable<T> container, T v) { return container.Count(x => EqualityComparer<T>.Default.Equals(x, v)); }
         public static void ForEach<TSource>(this IEnumerable<TSource> source, Action<TSource> actor) { foreach (var x in source) { actor(x); } }