formally 300 men, but only till around the social wars / -88 mainly
centuries later they even had 1200-1500 men strong legions, mainly mounted. but not that much in Caesars time. In mind the AUX are dominating, since they were thousands.
as Caesar you have three options to get mounted mercenaries from, German, Numbian and Gauls. while you have to many Aux in provinces which use the same counter than Roman cav.
even if not saying it was worthy cav. the romans had few hundreds of "roman"riders as recon force, antecursores, exploratores, not always mounted, it seems
not sure, but i think in Bibracte it was truly ROMAN cav. which lost.
while most later battles of the civ. war where Numidians/Gauls
...not paid by AGEOD.
however, prone to throw them into disarray.
‘Everything is very simple in War, but the simplest thing is difficult. These difficulties accumulate and produce a friction which no man can imagine exactly who has not seen War . . . in War, through the influence of an infinity of petty circumstances, which cannot properly be described on paper, things disappoint us, and we fall short of the mark.‘