Tiredofgaggs is right, study before casting.
regarding your first question of casting too many spells, yes and no. you can cast too many spells, this can leave you drained, possibly make you sick. spells need time to work, and your body needs time to get more energy. wait a while between casting. if you're casting over a person for the same thing, i personally wait one lunar month before casting another spell for the same thing. if you're casting a luck spell, health spell and a money spell on someone, i'd cast the most important one first, then wait a few days [maybe a week] before casting the next spell. if i felt up to it i might cast more than one spell, but spells take a lot of energy. when i know there's spell casting [or my coven's using the Ouija Board] i keep sweets near by otherwise a lot of my coven members energy crashes and they tend to fall asleep. just figure out your limit, eventually you'll be draining yourself and your spells will be pointless.