Seed beads come in sizes 1/0 to 24/0, although I don't think you can really buy size 24/0 any more.
The number of beads to the inch has NOTHING to do with their size.
Repeating from an earlier post, the usually accepted explanation for seed bead sizes is that there used to be a standard bead size, which was called the null bead, or 0 bead. Larger size beads were 1's, 2,'s, etc. When they started making smaller sizes they added zeros as they got smaller -- a 00 bead, a 000 bead, a 0000 bead, a 00000 bead, etc. Since it gets a bit tedious to write and count zeros, they began to refer to beads by the number of zeros they had, so a
000000 bead became a 6/0 bead, a 00000000 bead is an 8/0 bead, etc.
There's a chart at
formatting link
that covers 6/0through 15/0.