class YourForm : Form
{
private System.Windows.Forms.TextBox[] textboxes;
....
int n = 10;
int nLeft = 10;
int nTop = 10;
int nHeight = 30;
textboxes = new TextBox[n];
for (int i=0; i < n; i++)
{
textboxes[i] = new TextBox();
textboxes[i].Name = "txt" + i.ToString();
textboxes[i].Location = new System.Drawing.Point(nLeft, i*nHeight + nTop);
this.Controls.Add(textboxes[i]);
}