Will my extra sheet be checked if I forgot to write the number of?
Yes, definitely. The purpose of writing the number of extra sheets taken on the main answer book is to make the valuer aware of the fact that supplementary copy is attached too, which needs to be checked. It may happen that the thread gets loosen and the extra sheet is separated from the main one. Mentioning it in front helps to avoid the extra sheet remaining unchecked. So dont worry, your extra sheet will get checked and valued. Not writing it in the front wont hamper your marks.