Startup school companies were required to apply to the YC main program in order to be eligible for the $15k grant, and there were 30k companies that were part of SuS in 2019.
Also, from the figures presented at the conclusion of the program last year, there were ~23k applications to the 2019 summer & winter batch in total.
So roughly 10,500 then? These figures were shown at demo day?
There wasn't a specific breakdown, but I'd guess so. You can find the presentation here: https://www.dropbox.com/s/cq1fqp1zlkza3rf/SUS%202019%20By%20...
~200 companies a batch and supposedly a 2-3% acceptance rate puts it at 6,600-10,000.
They stopped updating their statistics: https://twitter.com/sama/status/618247698378985472 and https://blog.ycombinator.com/yc-portfolio-stats/
If it kept growing they would have proudly put that everywhere.
At the same time they introduced startup school which got 15,000 applicants, many of whom would have applied to Y Combinator but chose the startup school instead.
My bet is it peaked around ~8,500 and decreased when they introduced the startup school program.