What are the odds of a 23yo female having a brain tumor. are they rare?

Yes: They're rare. You can stop worrying, or at least find something else to worry about. It'd be much more productive to worry about the odds of unwanted pregnancy, sexually transmitted diseases, the dangers of smoking, or the risk of diabetes, to name but a few.

Very low odds: Unusual for a brain tumor to present initially in 20's, but some congenital lesion could have been present for years and then discovered by chance on an MRI study. A variety of problems could be considered, most quite benign. If lesion present, get consult from reputable neurosurgeon.