...and so on for all the other numbers. HOWEVER,
that is no-longer true. The numbers are still in use, but they dont match up anymore to real inches, and theyre a little different from brand to brand.
The best thing to do when youre employing beads in a situation like cross stitch or crochet where theyll need to fit into a grid is to
test them for gauge first. Cut a length of thread (at least a foot long else its hard to manage) and mark out exactly ONE INCH in the middle with a felt-tip pen or marker, like this:
thread ----------------------|------------|-------------------------
mark mark
...then string your beads on and count how many it takes to fill that inch.
If youre stitching on 14-ct (per inch) fabric for example, youll theoretically want beads that fit 14 to an inch... BUT, in personal experience, its best to use beads that are even a little smaller, like 15 or 16 to an inch. This allows for any minute variation, tensions, etc.
2) UNIFORMITY
Again, when youre putting beads into a grid where size is critical, shape will also be critical. Look for beads that are VERY uniform in shape. These, unfortunately, tend to be the slightly more expensive ones... not the cheap craft store kind, although sometimes you can find pretty good beads in a craft store bunch if youre willing to weed out the odd ones.
Two brands to look for are Miyuki brand beads (Delica is their most popular line), or Toho brand beads. Both are Japanese companies that keep their beads to VERY fine uniformity. You can get them online or in many bead stores (theyre pretty much known to be the best choice for beadweaving, too).
Here they are: www+artbeads+com/delicas-seed-beads.html (where + = .)
Good luck!