multithreading in objective-c - what limits?

i haven't started to learn objective-c yet. still learning c, in preperation for learning objective-c.

i plan to do something that'll require pretty heavy use of multithreading.

what are the limits, if any on multithreading? i mean, can you have thousands or are you restricted to a handfull? is it just the machine's power that ends up restricting you or are there more specific restiction within objective-c?