If you're cutting 1/8" or heavier stock, and most of what I cut with a jeweler's saw is at least that, use the coarsest blade, #8. The only thing I use finer blades for is cutting mosaic pins.
I've also found that like belts, buying expensive blades is cheaper in the long run. The old tool-buying rule of thumb to get the model that's one down from the top of the line seems to work for me on jeweler's saw blades. I can't remember the trade name for them but I buy the next-to-best (as in next most expensive) blades from Rio Grande. Since I've been doing that I've broken far fewer blades and they stay sharp much longer than the cheapies I used to buy. Be sure to use some kind of blade lube like beeswax.
It's worthwhile to buy blades in large quantity, I usually buy 144 at a time. That lasts almost forever now that I'm using better blades.