Magic items can be carried by characters and, in some cases, by the Standard Bearer of a unit of troops. This is indicated in the army list for each army in the appropriate Warhammer Army book. A character can only have one magic item, of each type (weapon, armour, etc) unless otherwise indicated.
No specific magic item can be carried by more than one model in the army, with the exception of scrolls. These are not limited in this respect and several scrolls of the same type can be carried by the same character if you wish.