Event is not triggering

I create one set it contain three column in the 1st row with seven texbox having the name with Amount and 2 column with textbox(7) as qty and 3 column as totalamount(7).in the !st row i i will add the amount by coding.Now in the quantitytextbox  i enter the amount and press enter means i got this error


Multiple controls with the same ID 'txtamount7' were found. FindControl requires that controls have unique IDs.

My code

 void amounttextbox()
   {
   
        int number = 7;
        for (int i = 1; i <= number; i++)
        {
            TextBox txtamount = new TextBox();
            txtamount.AutoPostBack = true;
      &nbs ...