hi micah, 

Kindly refer below sample.

Database

I have made use of the following table Customers with the schema as follows.

I have already inserted few records in the table.

You can download the database table SQL by clicking the download link below.

Download SQL file

HTML

<asp:TextBox ID="txtrecipt" runat="server" CssClass="form-control" placeholder="Recipt" ReadOnly="True"></asp:TextBox>
<asp:Label ID="lblrecipt" runat="server"></asp:Label>
<asp:LinkButton ID="btnrecipt" runat="server" CssClass="btn btn-default" OnClick="SetId">Call Receipt</asp:LinkButton>

Namespaces

C#

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

VB.Net

Imports System.Data.SqlClient
Imports System.Configuration

Code

C#

protected void SetId(object sender, EventArgs e)
{
    string connStr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    string previousIdQuery = "SELECT MAX(CustomerId) FROM Customers";
    txtrecipt.Text = GenerateNewId(connStr, previousIdQuery);
    string insertQuery = "INSERT INTO Customers (CustomerId) VALUES(" + GenerateNewId(connStr, previousIdQuery) + ")";
    using (SqlConnection con = new SqlConnection(connStr))
    {
        SqlCommand cmd = new SqlCommand(insertQuery, con);
        con.Open();
        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();
        if (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

Protected Sub SetId(ByVal sender As Object, ByVal e As EventArgs)
    Dim connStr As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString
    Dim previousIdQuery As String = "SELECT MAX(CustomerId) FROM Customers"
    txtrecipt.Text = GenerateNewId(connStr, previousIdQuery)
    Dim insertQuery As String = "INSERT INTO Customers (CustomerId) VALUES(" & GenerateNewId(connStr, previousIdQuery) & ")"
    Using con As SqlConnection = New SqlConnection(connStr)
        Dim cmd As SqlCommand = New SqlCommand(insertQuery, con)
        con.Open()
        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()
        If dr.Read() Then
            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 If
        con.Close()
    End Using

    Return String.Concat(newId)
End Function

 Screenshot