Spells are allocated to wizards at the start of the battle, before the first turn. You will only require one copy of Warhammer Magic; the set contains enough spell cards for both sides to use.
Generally speaking, most wizards use the spells from the Battle Magic spell deck. There are 20 cards in this deck, including repeats of the most useful spells. This means that even if no other spells are used there will be plenty to equip your wizards.
Some races have their own magic which they can use instead of Battle Magic. A few races must use their own magic (Orcs, Goblins and Skaven for example), but other races have a choice (High Elves and Chaos for example). A wizard who is allowed a choice must have either Battle Magic spells or his own racial magic spells but he cannot mix the two. We shall return to the special magic decks later, but for now we'll assume that wizards are using the Battle Magic deck.
Allocate spells at the start of the game as follows:
Each player declares the level of each wizard in his army.
The Battle Magic card deck is shuffled and dealt out face down to the wizards. Each wizard receives one spell for each magic level - these will be the spells that the wizard has for the duration of the game.
If there are insufficient spell cards to go round, deal to the highest level wizards first and then the lowest. Where necessary, the lowest level wizards must dice to decide which of them receives the remaining spells.
Once spells are allocated they remain fixed for the duration of the game. Wizards cannot swap spells between themselves, nor may they exchange spells they have been dealt. Each wizard's pile of cards must be kept distinct. If several wizards are involved on each side it is recommended that players note down on their roster sheets which spells belong to which wizards.
Note that where there are insufficient spells to equip all wizards some will go without or have fewer than normal. This is fine and serves as a natural limit on the amount of magic in the game.
For example: a High Elf army is fighting against an Empire Army. The High Elves have a level 4 and a level 2 wizard whilst the Empire has a level 4 and a level 1 wizard. All elect to use Battle Magic (the High Elves have the choice of Battle Magic or High Magic) and the cards are dealt between them. The level 4 wizards receive four spells each, the level 2 wizard receives two spells and the level 1 wizard one spell.