The breaking load is defined by the weakest link. Often this is a shaft or shackle and as such you don't get much variation with number of sheaves for a given size block. Max working load will be a percentage of this based upon a safety factor and as Ncik mentions expectations of non-destructive (elastic) deformation.

The only problem with Ncik's calc is it doesn't take into account the extra holding power provided by the ratchet and/or cleat. I.e. when you're in conditions where you couldn't pull the sheet on but because of technology you can hold it. Having said that.... you wont see anywhere near 650kg on your mainsheet, tornados don't get loads that high.