It will have multiple trunks, usually, and it looks like yours does. I like them that way. You can train it to a single trunk, but the blossom display wouldn't be as full and nice. It will naturally grow as a shrubby tree.
Your magnolia looks to be star magnolia cultivar which will grow to about 15-20' tall with a spread of 10'. If it's a saucer magnolia aka "tulip magnolia", it will grow 30' or more withe a similar spread.
Enjoy it!
