In a thesis statement, you need to include the reasons that will support you claim.
Try asking asking yourself the basic questions of: why? how? when? where? who? what?
If it is very an essay, I always try to stay at three reasons, as it usually helps to keep stability within the paper instead of making it sound choppy or too wordy. Make sure to always expound on your reasons and give lots of detail.
Remember: Evidence, and to try and use other methods such as Ethos, Pathos, and Logos