1. If you change professions and you don't have all the spells on a level (I'm guessing you've got either Malikto or Kadorto when you change), you will always get every spell in the level. 2. If you are not in a profession that uses those spells, you will have exactly the number of spells that are in the level.
PROTIP: Try leveling a priest to @lvl. 13 or so, change him to a mage. Repeat process. Change to a Bishop. Watch as fun ensues. You'll gain spell numbers slowly, but eventually you'll have a basic bonus priest/mage character that can identify items.
You'll be fairly old (25-6), but if you have Wizardry 2, you can transfer the character to that scenario and - if you find a few Stones of Youth - make him 19 again.