Disks are assigned numbers in the order found by the hardware. In the case of i386 or amd64 architecture, this is determined by the BIOS.
Assuming your system is i386 or amd64:
Your BIOS is in charge of picking the disk that is used to boot an OS. If you've told your BIOS to do that properly, you will get a boot> prompt.
At that boot> prompt, you can issue the "machine diskinfo" command to see what hard drives are detected, and then issue a "boot" command to select the specific device with a BSD disklabel partition and kernel. If you boot in single-user mode (hint: using a "boot" command at the boot prompt with the "-s" option), you can then mount enough of your partitions to be able to run an editor and edit your /etc/fstab as necessary, and add an /etc/boot.conf if necessary.
To understand both the commands you may type at the boot> prompt, and what an /etc/boot.conf file is used for, start with the
boot(8) man page.