For this you need to insert the number in database. On refresh retrieve the data from database and increment the value.

Check the below sample code.

SQL

CREATE TABLE GenerateUniqueId
(
    ID INT IDENTITY,
    UniqueId INT NOT NULL
)

Namespaces

C#

using System.Configuration;
using System.Data.SqlClient;

VB.Net

Imports System.Configuration
Imports System.Data.SqlClient

Code

C#

string connStr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
private void Page_Load(object sender, EventArgs e)
{
    string previousIdQuery = "SELECT MAX(UniqueId) UniqueId FROM GenerateUniqueId";
    string newId = GenerateNewId(connStr, previousIdQuery);
    lblId.Text = newId;
    string insertQuery = "INSERT INTO GenerateUniqueId VALUES(" + newId + ")";
    using (SqlConnection con = new SqlConnection(connStr))
    {
        con.Open();
        SqlCommand cmd = new SqlCommand(insertQuery, con);
        cmd.ExecuteNonQuery();
        con.Close();
    }
}

private string GenerateNewId(string connection, string query)
{
    string newId = string.Empty;
    using (SqlConnection con = new SqlConnection(connection))
    {
        con.Open();
        SqlCommand cmd = new SqlCommand(query, con);
        SqlDataReader dr = cmd.ExecuteReader();
        while (dr.Read())
        {
            string i = dr[0].ToString();
            if (string.IsNullOrEmpty(i))
            {
                newId = "1";
            }
            else
            {
                int j = Convert.ToInt32(i);
                j = j + 1;
                newId = j.ToString();
            }
        }
        con.Close();
    }

    return string.Concat(newId);
}

VB.Net

Private connStr As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString
Private Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    Dim previousIdQuery As String = "SELECT MAX(UniqueId) UniqueId FROM GenerateUniqueId"
    Dim newId As String = GenerateNewId(connStr, previousIdQuery)
    lblId.Text = newId
    Dim insertQuery As String = "INSERT INTO GenerateUniqueId VALUES(" & newId & ")"
    Using con As SqlConnection = New SqlConnection(connStr)
        con.Open()
        Dim cmd As SqlCommand = New SqlCommand(insertQuery, con)
        cmd.ExecuteNonQuery()
        con.Close()
    End Using
End Sub
Private Function GenerateNewId(ByVal connection As String, ByVal query As String) As String
    Dim newId As String = String.Empty
    Using con As SqlConnection = New SqlConnection(connection)
        con.Open()
        Dim cmd As SqlCommand = New SqlCommand(query, con)
        Dim dr As SqlDataReader = cmd.ExecuteReader()
        While dr.Read()
            Dim i As String = dr(0).ToString()
            If String.IsNullOrEmpty(i) Then
                newId = "1"
            Else
                Dim j As Integer = Convert.ToInt32(i)
                j = j + 1
                newId = j.ToString()
            End If
        End While
        con.Close()
    End Using
    Return String.Concat(newId)
End Function