Is their a large need for preschool/elementary school music teachers?

I agree with everything that BJ says!

Especially the playing the piano and singing! Learn to do both well! In college you will need to pass a piano proficiency exam, no matter what your major instrument is. And kids learn best by hearing the human voice, so learn to sing and practice your solfege! :)

The availability of jobs does depend upon the state. I'm from Texas and I recommend this state's music programs. They are amazing!!

While it depends on the state/area in which you plan to work, there are generally more general music jobs (especially elementary) than instrumental jobs. Having said that, instrumental teachers seem to move from district to district early in their careers until they find the perfect fit, so those jobs do tend to open and re-open more often.

Since solid teacher training for an instrumental music teacher should include most of what general music teaching requires (the foundations are the same, as are many of the methodologies), your best bet is to be prepared to work at any level. That will open your options for jobs when you graduate. It's a good idea to think about what specific age group you might want to specialize in, but don't let that give you an excuse for not becoming an excellent music teacher all around. You might also find that as your college experience progresses, you'll change your mind about your age preference (I know this from personal experience).

If you do decide to focus on elementary and/or general music, get your piano AND guitar chops up, and be prepared to sing - A LOT. Also, start thinking about a professional wardrobe that will allow you to move and dance for 7 hours a day :)

Bottom line = Focus your efforts to be great at performing, great at best practices for teaching, and great at being understanding and relatable to your students. If you do that, you'll be successful no matter what level or position you accept.
Best of luck!