Hi Asifpatel,

I have created sample code by refering the below article which full-fill your requirement.

ASP.Net ListBox: Save Multiple Selected Items in the Database

HTML

<form id="form1" runat="server">
<div>
    Employee Names:
    <br />
    <asp:ListBox ID="ListBox1" runat="server" Height="149px" SelectionMode="Multiple"
        Width="113px">
        <asp:ListItem>Vinz</asp:ListItem>
        <asp:ListItem>Jhen</asp:ListItem>
        <asp:ListItem>Chris</asp:ListItem>
        <asp:ListItem>Shynne</asp:ListItem>
        <asp:ListItem>Chu</asp:ListItem>
        <asp:ListItem>Mark</asp:ListItem>
        <asp:ListItem>Lilian</asp:ListItem>
        <asp:ListItem>Rod</asp:ListItem>
        <asp:ListItem>Glendzy</asp:ListItem>
    </asp:ListBox>
</div>
<br />
<asp:Button ID="Button1" runat="server" Text="Save" OnClick="Button1_Click" />
</form>

C#

private string GetConnectionString()
{
    return System.Configuration.ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
}

protected void Button1_Click(object sender, EventArgs e)
{
    string strEmployeeName = string.Empty;
    foreach (ListItem item in ListBox1.Items)
    {
        if (item.Selected)
        {
            strEmployeeName += item.Text + ",";
        }
    }
    InsertRecords(strEmployeeName.Remove(strEmployeeName.Length - 1));
}

private void InsertRecords(string employeeName)
{
    SqlConnection conn = new SqlConnection(GetConnectionString());
    SqlCommand cmd = new SqlCommand("INSERT INTO EmployeesTest Values(@EmployeeName)", conn);
    cmd.CommandType = CommandType.Text;
    cmd.Parameters.AddWithValue("@EmployeeName", employeeName);
    conn.Open();
    cmd.ExecuteNonQuery();
    conn.Close();
    Page.ClientScript.RegisterClientScriptBlock(typeof(Page), "Script", "alert('Records Successfuly Saved!');", true);
}

Screenshot