I only keep one nandroid on my SD card. Why keep more? I have flashed lots of ROMs and can set everything up the way I like it fairly fast, even if I don't use titanium (though that speeds things up). So I delete all nandroids before flashing a ROM, backup so i can revert if I don't want to use the new one or it causes problems, and flash away. I have no idea what people save several for.