I got a bunch of maple bonsai tree seeds, can i just plant a couple and care for them like normal plants? Will they only grow a certain size or do i need to do something to keep them small(like 12-18" would be best)?
"Bonsai" trees are not a speices of tree. It's viturally ANY tree that has been specially trained and pruned to be a small-in-stature, but mature-looking tree.
So what you have are Maple tree seeds, of some variety. Possibly marketed for bonsai work, since you need to start with a seedling, or otherwise extremely young tree to do good bonsai work.
It will NOT stay small, unless you train it. It involves careful pruning and shaping of branches (with stiff-but-malliable copper wire), and periodic root pruning, to keep the root structure small, which in turn dwarfs the tree.
It's an skill, and an art, which takes time and practice. I very much suggest reading some books on it, or even a beginners instructional video. It can be fun - but terribly frustrating if you don't know the basics!