You need to set range, which you want to select.
Example:
object what = WdGoToItem.wdGoToPage;
object which = WdGoToDirection.wdGoToAbsolute;
object count = 0;
const string fileName = "C:\\1.docx";
object fileNameAsObject = fileName;
Application wordApplication = new Application();
object readOnly = false;
object missing = System.Reflection.Missing.Value;
wordApplication.Documents.Open(ref fileNameAsObject, ref missing, ref readOnly, ref missing,
ref missing, ref missing, ref missing, ref missing,
ref missing, ref missing, ref missing, ref missing,
ref missing, ref missing, ref missing, ref missing);
Range startRange = wordApplication.Selection.GoTo(ref what, ref which, ref count, ref missing);
object count2 = (int)count + 1;
Range endRange = wordApplication.Selection.GoTo(ref what, ref which, ref count2, ref missing);
//if you want to select last page
if (endRange.Start == startRange.Start)
{
which = WdGoToDirection.wdGoToLast;
what = WdGoToItem.wdGoToLine;
endRange = wordApplication.Selection.GoTo(ref what, ref which, ref count2, ref missing);
}
endRange.SetRange(startRange.Start, endRange.End);
endRange.Select();