Do all javascript webworkers for a page operate on a single separate thread from the main page, or does each webworker get its own thread? For context: I have a large amount of data that is currently being processed by one webworker. Would it be faster to split it up across 10 webworkers, or will they all use the same thread?
That is not defined by the standard:
This standard does not define the precise mechanism by which this is achieved, be it time-sharing cooperative multitasking, fibers, threads, processes, using different hyperthreads, cores, CPUs, machines, etc.
Even if each webworker gets its own thread, the CPU is not able to run them in parallel, if the number of cores is lower then the number of web workers. Also other threads are using the CPU.
Web workers have not exactly been made for you scenario. They are more of a 'don't block the UI' thing. See also this statement from the standard:
Workers (as these background scripts are called herein) are relatively heavy-weight, and are not intended to be used in large numbers
The only sensible thing to do is to test what works best (with all relevant browsers). You can check self.navigator.hardwareConcurrency (see here) in your code to decide how many workers you should spawn.