It would be an exceptionally bad choice to use a host that has such a pricing structure, most will just fail with no extra costs.

Otherwise, well obviously a small VPS will not handle the load from hundreds of thousands of simultaneous viewers, that's just how it is. How many it can handle is unclear though, I don't think Peertube has been stress tested under real world conditions like that. However the p2p offloading is fully client side and does not involve the server directly, thus it should at least to some extend help.