The surveyed Wikipedia treatment states that Taiwan's Indigenous peoples are about 2.3 percent of the island's population and that only 35 percent of them speak their ancestral language, but gives no combined aggregate count of Formosan-language speakers, and the two percentages alone are not a citable total.
What would resolve this A published aggregate count of living Formosan-language speakers across Taiwan's officially recognized Indigenous peoples, from a source such as Taiwan's Council of Indigenous Peoples or a UNESCO endangered-languages survey.
OpenAustronesian linguistics and language documentation (Taiwan).Wikipedia: Formosan Languages