How many bios rom chips are in a PC?

Usually one. If you're just talking about a motherboard BiOS. Even videocards, sound cards and indeed even network cards also have BiOS chips in some cases.

New board of a performance class (such as ASUS's Republic of Gamers ROG) boards have been known to have dual BiOS chips which allow the user to select which one of the chips they would like to use. This allows for performance tweaking and overclocking without risking being unable to boot due to corrupted setting OR due to a failed BiOS upgrade for example.

Hope this helps!