This item is from the Yet Another Enhanced IDE/Fast-ATA/ATA-2 FAQ, by John Wehman and Peter den Haan with numerous contributions by others. (v1.92).
Too many. See question 12.4 for a discussion of some of the differences between a Phoenix EBIOS and a WD EBIOS. For a more exhaustive discussion of BIOS types, Hale Landis' effort is indispensable---see the resource guide below.