I have been doing a lot of NAND back-ups the past couple days. I have 17 currently saved right now, I'm just curious how many every has to date, if you like to keep just a few or back-up EVERYthing.
I periodically back up my primary ROM every couple of days (or less) as I perform rooting experiments - after all, I want to come back to right where I left it.
Because that primary ROM is heavily configured and used, and probably has all sorts of trash in cache, it takes about 260 MB to back it up just once. So, for an 8 GB SD card, there would be a practical limit of about 30-32 of those backups - far less if I wanted to have an ext partition for A2SD, or music on the card, or space for photos, or ....
OTOH, I'm performing a backup only to have a copy of my most recent configuration, so I tend to delete old copies of the same ROM as I go along.
Also - truth be told - once you've had a quick peek at a ROM, what's the chance that you are going to ever re-load it? Seems like a ROM either becomes your fave, or it doesn't.
I suppose you could switch ROMs like you might switch wristwatches (as a fashion accessory), but after a while, keeping multiple ROMs configured so that all of your "stuff" (bookmarks, etc) are in there and up-to-date starts to become a hassle.
But, don't do what I say - just do what suits you. (Make sure to keep enough space on the SD card so that you can do "just one more" backup, though!)
eu1