Neither threads nor turfs are processes in the Unix sense, so pid_t can remain unchanged. A Unix process can be seen as an initial thread residing in a new turf, but you can have turfs without threads on the Mill, which is not possible on Unix because there is no way to have an isolated load module that is a pure service.
Stack segments can be arbitrary sized, subject to OS enforced quanta per usual. The info block describes the top segment of the segmented stack. A new service call (not a callback) will always use the reserved stacklet, but it initially has no data frame (spiller has a frame of course). Creating a frame is a distinct Mill operation stackf, which carries a size in bytes. If that exceeds the current limit then the hardware traps to a handler (which would ordinarily be a portal to a service) that runs out and allocates enough space for another segment, fixes up the links and the frame and stack pointer registers, and updates the info block. The handler enforces whatever quantum policy is desired. The alloca operation can also trigger stack overflow, with similar handling.
Subsequent callbacks will use the new segment. An exit followed by a new call can be optimized if the handler keeps a previous, now empty, segment around to avoid allocate/deallocate overhead when call/returns are bouncing over a segment boundary.