Conversion failed when converting the nvarchar value to data type int при подсчете записей

SqlCommand UsersCount = new SqlCommand("SELECT COUNT(*) FROM [Clients] WHERE [login] = " + TextBox1.Text, sqlConnection);
                    int count = Convert.ToInt32(UsersCount.ExecuteScalar());

using System;
using System.Collections.Generic;
using System.Linq;
using System.Data;
using System.Data.SqlClient;
using System.Configuration;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
namespace WebApplication1
{
    public partial class LoginSite : System.Web.UI.Page
    {
        public int idcookie;
        public string rules;
            private SqlConnection sqlConnection = null;
        protected async void Page_Load(object sender, EventArgs e)
        {
            string connectionString = ConfigurationManager.ConnectionStrings["DBConnection"].ConnectionString;
            sqlConnection = new SqlConnection(connectionString);
            await sqlConnection.OpenAsync();
        }
 
        protected async void Button1_Click(object sender, EventArgs e)
        {
 
            Dictionary<string, string> db = new Dictionary<string, string>();
            SqlCommand getUsersCredCmd = new SqlCommand("SELECT [login], [password] FROM [Users]", sqlConnection);
            SqlDataReader sqlReader = null;
            try
            {
                sqlReader = await getUsersCredCmd.ExecuteReaderAsync();
                while (await sqlReader.ReadAsync())
                {
                    db.Add(Convert.ToString(sqlReader["login"]), Convert.ToString(sqlReader["password"]));
                }
            }
            catch { }
            finally
            {
                if (sqlReader != null)
                    sqlReader.Close();
            }
           // try
            {
                if (TextBox2.Text == db[TextBox1.Text])
                {
                    SqlCommand UsersCount = new SqlCommand("SELECT COUNT(*) FROM [Clients] WHERE [login] = " + TextBox1.Text, sqlConnection);
                    int count = Convert.ToInt32(UsersCount.ExecuteScalar());
                    if (count > 0)
                    {
                        SqlCommand UserID = new SqlCommand("SELECT [id] FROM [Clients] WHERE [login] = " + TextBox1.Text, sqlConnection);
                        int userid = Convert.ToInt32(UserID.ExecuteScalar());
                        idcookie = userid;
                        rules = "Client";
                    }
                    else
                    {
                        SqlCommand UserID = new SqlCommand("SELECT [id] FROM [Drivers] WHERE [login] = " + TextBox1.Text, sqlConnection);
                        int userid = Convert.ToInt32(UserID.ExecuteScalar());
                        idcookie = userid;
                        rules = "Driver";
                    }
                    HttpCookie rulecookie = new HttpCookie("rules", rules);
                    HttpCookie id = new HttpCookie("id", idcookie.ToString());
                    HttpCookie login = new HttpCookie("login", TextBox1.Text);
                    HttpCookie sign = new HttpCookie("sign", SignGenerator.GetSign(TextBox2.Text + "bytepp"));
                    HttpCookie signrule = new HttpCookie("signrule", SignGenerator.GetSign(rules + "bytepp"));
                    Response.Cookies.Add(login);
                    Response.Cookies.Add(sign);
                    Response.Redirect("WelcomeSite.aspx", false);
                }
                else
                {
                    string script = "alert('Неверный логин или пароль');";
                    ClientScript.RegisterClientScriptBlock(this.GetType(), "MessageBox", script, true);
                }
            }
           // catch
            {
 
               //string script = "alert('Введите логин и пароль');";
                //ClientScript.RegisterClientScriptBlock(this.GetType(), "MessageBox", script, true);
 
            }
 
        }
    }
 }

SqlCommand reguser = new SqlCommand("INSERT INTO [Users] (login, password)VALUES(@login, @password)", sqlConnection);
                reguser.Parameters.AddWithValue("login", TextBox1.Text);
                reguser.Parameters.AddWithValue("password", TextBox2.Text);
                await reguser.ExecuteNonQueryAsync();
                //SqlCommand UserID = new SqlCommand("SELECT [id] FROM [Users] WHERE [login] = " + TextBox1.Text, sqlConnection);
                //SqlDataReader sqlReaderID = null;
                //sqlReaderID = await UserID.ExecuteReaderAsync();
                //int userid = Convert.ToInt32(UserID.ExecuteScalar());
                reguser = new SqlCommand("INSERT INTO [Clients] (name, surname, fathername, phone, login)VALUES(@name, @surname,@fathername,@phone, @login)", sqlConnection);
                reguser.Parameters.AddWithValue("name", TextBox3.Text);
                reguser.Parameters.AddWithValue("surname", TextBox4.Text);
                reguser.Parameters.AddWithValue("fathername", TextBox5.Text);
                reguser.Parameters.AddWithValue("phone", TextBox6.Text);
                reguser.Parameters.AddWithValue("login", TextBox1.Text);
                Response.Redirect("LoginSite.aspx", false);
                await reguser.ExecuteNonQueryAsync();

SqlCommand UsersCount = new SqlCommand("SELECT COUNT(*) FROM [Clients] WHERE [login] = '" + TextBox1.Text+"'", sqlConnection);