In my experience (so far), the first alternate superblock has always been 32. Beyond that it depends on the disk geometry, but you can get a list by running newfs (or mkfs) with the option to not really recreate a new filesystem and just print what it would do. With luck the defaults it picks for the disk geometry will match what was used when the filesystem was created, and it will give you a valid list of alternate superblocks. Additional superblocks may be moot though, because when things were bad enough to trash both my primary superblock and the first alternate (@32), my data was trashed too. And the disk was probably a candidate for replacement.
I save (elsewhere, several elsewheres) the output of newfs as part of my system-config info just for this sort of occasion.
Oh, the disk has been replaced (which is why (a) it's now in an external USB enclosure, and (b) I have a working Mac to poke at it from). I'm just hoping I can get back a few of the files that I wasn't able to back up in time before it became unbootable.
fsck
Additional superblocks may be moot though, because when things were bad enough to trash both my primary superblock and the first alternate (@32), my data was trashed too. And the disk was probably a candidate for replacement.
I save (elsewhere, several elsewheres) the output of newfs as part of my system-config info just for this sort of occasion.
Re: fsck