Yu gi oh magical arm shield?

my brother and i were playing a game of duel monsters / yu gi oh. he attacked me with a monster with 2000 atk points i currently and regenerating mummy and spirit reaper on the field, he attacked my regenerating mummy and i used magical arm shield he said i cant use it because i have to have exactly one monster on the field because the card says if you have A monster meaning only one. do you think he's correct and if i take control of a monster in his side of the field in atk position and the monsters attack is 1800 does ho lose thew life points or do i lose the life points because if u attack relingquish the equipped monster dies and the owner loses the life points