As far as I understood your question you want to display 20 records per page in gridview.
sorry if misunderstood.
Set the Griidview
AllowPaging="true"
and
pagesize="20"
In page index changing event write the following code
protected void Gridview1_PageIndexChanging(object sender, GridViewPageEventArgs e)
{
gvEMaterials.PageIndex = e.NewPageIndex;
FillGrid();
}