U.S. News provides “selectivity ranks,” which can offer you a rough sense of comparative selectivity:
- Bowdoin
- W&L
- Colby
- Colgate
- Middlebury
- Bates
- Oberlin
- Union
St. Lawrence: N.A.
It appears that your first six choices space fairly tightly, with significantly greater chances of admission beginning after that.
With respect to research into suitable choices for your intended major, you may want to seek the print edition of the Princeton Review college guide, which includes a sampling, “Great Schools for Mathematics Majors.” Highly selective LACs such as Pomona, Haverford, Amherst, Hamilton, Williams and Bowdoin appear, but so does the moderately selective St. Lawrence from your list.