How to get total element tag in C#?

That's because you only count and process lines containing "<L1" - The other elements in your sample start with "<H" or <"P" so they are ignored.

Quote:

Really, it's mean you tried it before. But the results like that or not?

No, but I just tried it: it's not a complex task.
Create a class to hold tag info:

protected sealed class TagItem
    {
    public int StartIndex { get; set; }
    public int EndIndex { get; set; }
    public int TagsCount { get; set; }
    public TagItem Parent { get; set; }
    public override string ToString()
        {
        return $"Start   : {StartIndex}\nEnd     : {EndIndex}\nContains: {TagsCount}";
        }
    }

Then create a List to hold them as you find them and a "current tag" variable so you know what a new tag belongs to:

List<TagItem> tags = new List<TagItem>();
TagItem currentTag = null;

Then look at each character in the input string (no need to Split it!) looking for start and end tags.

For a start, count it on the current tag (if there is one), create a new TagItem with the character index and current tag as its parent, then add that to the List and set the new current tag.

For an end, set the end index on the current tag, and set it back to it's parent.

After the loop, print counts for all the non-zero tags:

Console.WriteLine(string.Join(",", tags.Where(t => t.TagsCount > 0).Select(t => t.TagsCount)));

5 minutes of typing, or so?
Result: "2,1,3,1"

Seriously, the way I suggested is way, way easier!

        protected sealed class TagItem
            {
            public int StartIndex { get; set; }
            public int EndIndex { get; set; }
            public int TagsCount { get; set; }
            public TagItem Parent { get; set; }
            public override string ToString()
                {
                return $"Start   : {StartIndex}\nEnd     : {EndIndex}\nContains: {TagsCount}";
                }
            }
        /// <summary>
        /// Called once when form displayed for first time
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void FrmMain_Shown(object sender, EventArgs epp)
            {
            string str = "<L1\r\n <L1\r\n  <H1 content> \r\n > \r\n <L1\r\n  <H2 content> \r\n  <P content>\r\n  <L1\r\n   <H3 content>\r\n  >\r\n >\r\n>";
            List<TagItem> tags = new List<TagItem>();
            TagItem currentTag = null;   
            for (int i = 0; i < str.Length; i++)
                {
                if (str[i] == '<')
                    {
                    if (currentTag != null) currentTag.TagsCount++;
... 2 lines removed
                    }
                else if (str[i] == '>')
                    {
                    if (currentTag == null) throw new ArgumentException($"Input contains mismatched tags: unexpected \">\" at position {i}");
... 2 lines removed
                    }
                }
            Console.WriteLine(string.Join(",", tags.Where(t => t.TagsCount > 0).Select(t => t.TagsCount)));
            }

Now, you can't just hand that in - I removed a few lines of code - but you are really struggling with the way you are doing it because it's a clumsy way and you keep hitting it with a hammer to bend it around another edge case! Ask yourself this: "Why am I at all concerned with lines of text? Do they actually matter?"
If the answer is "No, they don't matter at all, but I thought they did and now I'm stuck with them" then think about throwing your existing way of doing it away and start again, with a plan for the end result in mind to start with.

Sometimes, the hardest thing to do is admit you made a mistake and the whole approach is flawed - but we all do it, and it's always a wrench to discard work we laboured on. You do get a "better" solution though!