For sufficiently small subfields, the number of jobs is small enough to count manually. For example, the HEP rumor mill counts all high energy physics jobs in the US. The number of jobs per year is in the very low double digits.
Are there statistics on the number of postdoc/professor positions per year in specific subfields? I'd be particularly interested in the fraction of interested undergrads that get into grad school in that subfield, the fraction of grads that get postdocs, the fraction of postdocs that get tenure-track positions, and the fraction of those professors that get tenure.
I'm aware that these statistics exist by scientific field, and the AIP has counts of graduate students by physics subfield, but I'd be interested in postdoc/professorship stats too.