However, the subject of the thread is a 24 HD aberration with 24 class levels. Thus the maximum number of attacks it will ever have is (arguably) three due to the aberration 3/4 BAB. Granted, aberrations only have one (or more) natural attacks, rather than BAB progression of attacks, but if the initial HD of the creature do not take precident, then - in theory - a 20 HD humanoid creature with 20 levels in wizard / etc would have 4 attacks (rather than the 3 typical of 3/4 humanoid BAB or the 2 typical of the wizard class.

Actually, now that I think about it, that is a bit of an odd situation, isn't it?
If a creature that was of the Humanoid type - with 14 racial HD and a single slam attck - took 11 levels of wizard, how many attacks would it have? Would it have 3 (the typical of a creature of that level with 3/4 bab), would it have 4 (as some arguments on this thread seem to suggest - that all pure creatures or 'more than a few HD' creatures that take enough levels in a class eventually have 4 attacks - even if this is not possible until into epic level), or would it have 2, the number expected of wizards?
It seems that the rules would state it only has 2 attacks, for its HD is one short of the necessary number for 3 attacks, and wizard levels do not grant the option of a third attack even in epic levels. On the other hand, if later the creature should choose to take a single racial HD - advancing himself in a racial level rather than a class level, suddenly he has 3 attacks - even if the base attack bonus was well beyond what is typically necessary for such prior to that racial level - and upon taking the 15th racial level is actually high enough to qualify for four attacks (+16) - if such is an option.
The whole situation with BAB is just a little confusing whenever class levels and racial levels mix.