这可能会有点棘手，因为不仅仅是字体的问题，例如，行间距会起作用，因为rtf中的每个CR都将是一个新的/par。

下面是计算字符位置并在Y坐标改变时生成新行标签的方案的第一遍。

public partial class RichTextBoxEx : UserControl
{
    const int 
        MAX_VISIBLE_LINES = 100,
        LINE_LABEL_HEIGHT = 20;
    public RichTextBoxEx()
    {
        InitializeComponent();
        for (int i = 0; i < MAX_VISIBLE_LINES; i++)
        {
            panel.Controls.Add(new Label
            {
                Name = $"label{i}",
                Text = "Placeholder",
                Location = new Point(0, i * LINE_LABEL_HEIGHT),
                TextAlign = ContentAlignment.MiddleLeft,
                Size = new Size(panel.Width, LINE_LABEL_HEIGHT),
                AutoSize = false,
            });
        }
        _labels = panel.Controls.OfType<Label>().ToArray();
    }
    private readonly Label[] _labels;
    protected override void OnCreateControl()
    {
        base.OnCreateControl();
        richTextBox.VScroll += recalc;
        richTextBox.TextChanged += recalc;
        richTextBox.Rtf = rtf;
    }

    private void recalc(object sender, EventArgs e)
    {
        int charIndex = richTextBox
        .GetLineFromCharIndex(
            richTextBox.GetCharIndexFromPosition(Point.Empty));

        int y, delta, cum = 0, prevY = 0;
        int labelIndex = 0;
        while(charIndex < richTextBox.Text.Length)
        {
            Point pos = richTextBox.GetPositionFromCharIndex(charIndex);
            y = pos.Y;
            if (!y.Equals(prevY))
            {
                delta = y - prevY;
                cum += delta;
                Debug.WriteLine($"{prevY} {y} {delta} {cum}");
                prevY = y;
                _labels[labelIndex].Top = cum;
                _labels[labelIndex].Visible = true;
                _labels[labelIndex].Text = $"{labelIndex + 1}";
                labelIndex++;
            }
            charIndex++;
        }
        while (labelIndex < MAX_VISIBLE_LINES)
        {
            _labels[labelIndex++].Visible = false;
        }
    }

    const string rtf =
@"{\rtf1\ansi\ansicpg1252\deff0\nouicompat\deflang1033{\fonttbl{\f0\fnil\fcharset0 Calibri;}}
{\*\generator Riched20 10.0.22621}\viewkind4\uc1 
\pard\sa200\sl276\slmult1\f0\fs22\lang9 Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed pellentesque, odio ac posuere vehicula, risus arcu cursus nunc, a varius nisi lectus fermentum quam. Maecenas dignissim metus sed felis sodales suscipit. Aliquam sagittis pharetra massa, eu hendrerit erat maximus non. Nulla facilisi. Quisque libero urna, vulputate ut maximus in, sodales vitae mi. \par
\b\fs40 Suspendisse\b0\fs22  \par
In est metus. Donec id turpis lorem. Curabitur id ipsum bibendum, fringilla eros vel, vulputate enim. Pellentesque nec sem a sem rhoncus ultrices. Quisque at purus a odio semper sagittis a at tellus. \par
}";
}

@IVSoftware有一个很好的答案，因为它考虑到每行的字体可能不同。
但是，如果整个文档的字体相同，则需要将事件分配给rtb的FontChange事件，或者在对话框中设置字体时将字体分配给两个RTBox。
您的RTBox似乎在RTLSourceView(...)方法的作用域之外不可用，因此要访问它，您需要知道控件在面板中的位置（或循环通过Controls属性）。RtlRichTextBox可以在初始化时实现某些默认字体，在您代码中，有一个RtlRichTextBox用于rtb，有一个标准的RichTextBox用于line_numbers。

public partial class Form1 : Form
    {
        //RichTextBox line_numbers;
        //RichTextBox rtb;
        public Form1()
        {
            InitializeComponent();

            DoubleBuffered = true;
            Dock = DockStyle.Fill;

            RichTextBox rtb = new RichTextBox();
            //rtb.Text = System.IO.File.ReadAllText(filename);
            rtb.Dock = DockStyle.Fill;
            rtb.ForeColor = Color.White;
            rtb.BackColor = Color.Black;
            rtb.Dock = DockStyle.Fill;
            rtb.TextChanged += Rtb_TextChanged;
            rtb.FontChanged += Rtb_FontChanged;
            splitContainer1.Panel2.Controls.Add(rtb);

            RichTextBox line_numbers = new RichTextBox();
            line_numbers.Multiline = true;
            line_numbers.Dock = DockStyle.Fill;
            line_numbers.WordWrap = false;
            line_numbers.ReadOnly = true;
            line_numbers.ForeColor = Color.White;
            line_numbers.BackColor = Color.Black;
            line_numbers.SelectionLength = 0;
            line_numbers.Font = rtb.Font;
            line_numbers.ScrollBars = RichTextBoxScrollBars.None;
            splitContainer1.Panel1.Controls.Add(line_numbers);
        }

        private void Rtb_FontChanged(object sender, EventArgs e)
        {
            if (sender is RichTextBox rtb)
            {
                if (splitContainer1.Panel1.Controls[0] is RichTextBox line_numbers)
                {
                    line_numbers.Font = rtb.Font;
                }
            }
        }

        private void Rtb_TextChanged(object sender, EventArgs e)
        {
            if (sender is RichTextBox rtb)
            {
                if (splitContainer1.Panel1.Controls[0] is RichTextBox line_numbers)
                {
                    line_numbers.Clear();
                    for (int i = 0; i < rtb.Lines.Length; i++)
                    {
                        line_numbers.Text += $"{i + 1}{Environment.NewLine}";
                    }
                }
            }
        }

        private void button1_Click(object sender, EventArgs e)
        {
            if (fontDialog1.ShowDialog() == DialogResult.OK)
            {
                if (splitContainer1.Panel2.Controls[0] is RichTextBox rtb)
                {
                    rtb.Font = fontDialog1.Font;
                }
            }
        }
    
    }

上面的代码只是一个表单，其中有一个按钮用于调用FontDialog.ShowDialog()和拆分面板。