Please help.

Intelligence determines a couple things: it's part of the criteria for jobs at the Job center, as well as determining which spells you can see in the Secluded Shrine, and how many spells your pet can memorize for the Battle Coliseum.

16 Intelligence points equals 1 Alchemy point, and most spells are around 7 alchemy points to use. You want more than that, so you can have a few different spells in use at any one time. My pet Maebh is using 33 of her 46 alchemy points, for example, on 5 different spells so I have a good range.