You've already forked cpp-httplib
CPPHTTPLIB_MULTIPART_FORM_DATA_FILE_MAX_COUNT is enforced only in Server::read_content(), where parts are accumulated into req.form. The streaming ContentReader path keeps nothing and was never in scope, but this was undocumented (GHSA-923p-8q8g-xcqj). Note the split in the README and show how to bound the part count from inside a ContentReader handler.