You can use just about any number you like:
The created groups are not part of any standard—they are groups decided on in part by the requirements of the Udev configuration in this chapter, and in part by common convention employed by a number of existing Linux distributions. The Linux Standard Base (LSB, available at http://www.linuxbase.org) recommends only that, besides the group root with a Group ID (GID) of 0, a group bin with a GID of 1 be present. All other group names and GIDs can be chosen freely by the system administrator since well-written programs do not depend on GID numbers, but rather use the group's name.
I've created a (lfs) system which had uid/gid 0 and 1 (both: root/bin), all the rest was made up by me.
I did follow one other "rule": uid's for users start at 1000 (there are actually 2 "rules" for this one, the other states you need to start at 500).
One does need to be aware that the book does sometimes use uid's/gid's that have been created in earlier chapters.