As dictator, shouldn't Caesar have had at least 24 lictors protecting him at all times (since Sulla had dispensed with the stipulation that only 12 lictors were to accompany a dictator within Rome, I assume Caesar likewise ignored that restriction)? If the lictors were there, how did the senators manage to not only kill Caesar, but not be killed themselves right afterward? If the lictors weren't there, why weren't they there?
First, be aware that starting with Julius' predecessor, Sulla, the "Lictor rule" was ignored at a whim by the dictator as you pointed out. While I couldn't find a reliable source for this, I did find many that speculated Julius wanted to be perceived every bit as badass as Sulla and tried to ignore the lictor rule 'just as hard' as he did. He frequently had only a couple lictors with him and he often ordered them to trail behind him rather than go in front.
In 'Lives of the Twelve Caesars' Suetonius wrote that on the day of his death, Caesar was accompanied by lictors (possibly as few as 2 or as many as 4) and announced as per usual as he was brought into the Senate chamber. He was then surrounded by a huge group of 60 or more individuals all coming to 'pay respects'. This group contained the conspirators and they probably visually blocked out the lictors as the violence began. What happened to the lictors at this point is unclear but it is likely that they fled. Weapons were not allowed at all in the Senate except for lictors, and a giant mob of people breaking out 23 or more weapons suddenly was likely not something 2 appointed people wanted to stick around for.
It's also entirely possible they were paid off; we simply don't know.
Bear in mind, the normal job of a lictor is to announce, or to shove matrons out of the way when ordered to do so. They are not the praetorian guard (at least not in times or places of peace). They carried a rod and/or an axe and had just as many ceremonial duties and roles as they did practical bodyguarding duties in a place like the Senate. A lictor was a somewhat cushy job that centurions retired to in order to garner additional social standing; not a life-oath to protect a Caesar.
Maybe it would be helpful, if someone could detail what being accompanied by a lictor actually means:
How far would the be away? Where the always around or just on the street? ehat about in private homes? Where would the 24 lictors of the consuls be during a normal senat session?...
Having 24 (or even 12) guys standing constantly around you would be quite disturbing at any kind of political or social gathering.