Each (physical) disk formatted to be a boot device will have a (valid) MBR.
However, BIOS, due to its limitations, scan all device in the boot order (specified on boot parameters), seeking out for a good mbr. The first one found will be use as boot device.

On some bios will can open a boot menu (with F8) and choose from where you want to boot.

MBR, superblocks and partition table are part of disk formatting structure.

As an example, if you install 2 HDD on a machine and install Windows, Windows installer will install the O.S. on the 1st disk (by default you can choose). The 2nd disk will not contain a valid MBR.