CPSLO’s career survey is at https://careers.calpoly.edu/search.php .
It looks like few math majors there go on to PhD programs in math (about 1 per year out of 40 respondents to the survey per year).
UCSC’s career survey is at http://careers.ucsc.edu/about/fds_reports_page.html .
For math, it looks like 42% (8 out of 19) will go to graduate study within a year of graduation, although it looks like about a third of graduate study plans are in math or statistics.