Depends on the app/framework but typically phantom threads don’t get started. Also, those stacks aren’t materialized until you write something - they’re all CoW’ed pages from a sentinel 0-filled page the kernel keeps around just for this purpose.
The stacks are allocated gradually, but there's a minimum few committed pages for each thread and there can be a lot of threads. The graphics driver alone will typically spawn a thread per CPU core unless you specifically disable threading optimizations, and that's separate from the thread pool.
