It seems you never got a definitive answer on this topic. I have read several sites that have said there were 18 vestals – with 6 as acting vestals at any given time – and yet none of these actually cites any ancient source or archaeological evidence to show that this is true. I think it might just be an assumption based on what we do know – that there were periods of 10 years each for training, service and teaching. My guess is that is just a delineation of the general path of a vestals' service, not an annunciation that there were discrete classes of vestals with 6 in each one. Let's consider the evidence: While I've found no ancient source that mentions 18 vestals, other well established facts about the vestals seem to contradict the idea of 18. For one, in all of the mentions of individual vestals (whether through illustrious deeds or trials and punishment), none are ever mentioned as being novices in training, or "acting vestals" or serving as senior trainers. They are all simply vestals, in other words, all having the same status and subject to the same rules and punishments. Surely the rules on vestals' conduct pertained to all vestals regardless of whether they were in training or not. Second, we know that the virgo maxima (the vestal in charge) was the senior most vestal. If that is the case, then she had completed her service of 10 years as "acting" vestal. And yet the virgo maxima is mentioned in ancient sources as being key to leading various religious rites. She's hardly in the background teaching the young. Third, we have heard a considerable amount about the captio of the young vestals and the rites that followed as they were inducted into the college. Why is there no mention of a novice vestal "graduating" into the class of active vestal? Considering how deeply ritualistic the Romans were, this seems like it would be a thing and yet, no mention. Finally, we know of at least one case where 3 vestals were condemned and put to death at a time (83 CE under Domitian). This would've left just 3 vestals, obviously a big problem. To my mind, this explains why the vestals who died prematurely could be replaced by novices who were not the usual 6-10 years of age, sometimes even by women who were not virgins. Why else would they bend such a sacrosanct rule if there were 18 vestals? If you had 18 and 3 were put to death – even if all 3 were in the middle group of "active" vestals, you'd simply elevate the most senior of the young vestals and start 3 more in training. But if you only had 6 total and lost 3, well then, it makes sense that you wouldn't want fully half of your college to be 6-10 year olds. Also, I don't know what the hell 18 vestals would be used for. They didn't have that much to do, frankly. Plus, in every contemporaneous image of the vestals, there are always 5 being shown. Why? Because one was always tending to the fire of Vesta. If there were 18, that would not be necessary. Anyway, that's my thinking. If anyone has found some actual evidence that I'm wrong, and that there actually were 18 vestals in the college, I'd appreciate it This is central to the plot of a book I'm writing and I'd like to get it right. Thanks much!