Try this:
//get from db and put text-box dynamically as below:
Database object loop{
TextBox new_textbox = new TextBox();
new_textbox.ID = "txt" + 1;
new_textbox.Text = "";
PlaceHolder1.Controls.Add(new_textbox);
}
When you click submit button then need to get value of all text box, so for that again do same as above:
Database object loop{
TextBox tb = (TextBox)PlaceHolder1.FindControl(dbobject.[name]);
string str1 = tb.text;
}