I do not have any numbers on that. The best source of statistics on immersion is the annual report of Canadian Parents for French. This organization does very patient work in collecting data on various aspects of immersion.
Here is an anecdote. In west Toronto, one of the parents told me that the school board district to which his children's school belonged was organized along a north-south axis. He said that children living in the low-cost housing in the Jane-Finch neighbourhood took long trips by bus every day to go to an immersion school because their parents know that bilingualism is a factor in advancement. As to whether this happens elsewhere, I cannot tell you.
You touched on an important point. The immersion system is often criticized as being only for the elite even though it is funded by taxpayers. This is a frustrating criticism. If students in an immersion program have learning disabilities that have nothing to do with the language of instruction, the school's first reaction will be to pressure the parents of these children to take them out of the program. After a few years, in grade 7, grade 8 or grade 9, all children with learning disabilities are placed in regular classrooms. Because of decisions made by the schools, only those students who excel will be in an immersion program, because those who did not will have been excluded from the system. The system is then criticized and said to be for the elite only. I find this criticism deeply unfair.