Never or the same moment they have arrived.
They have the technology of recreating production capacity, then it more or less straightforward push the button thing, and just takes time to locate additional resources if they require expansion.
if they do not, then even keeping 1800's tech will be problematic over the 100+ years.
I would say it takes about 20 differently skilled people to raise one skilled guy, based on my past experience. Specializations do overlap - like basic math is useful in all sort of fields but there are different fields of math which are more specific and each needs a guy, fundamental knowledge about chemistry and then I would say you need at least one guy for each of about at least 10 specializations of chemistry which one needs for industry, physics - school level that is one guy, but then flavors of it to which there is no end.
The number of different knowledge required to keep wheels rolling for the current technological system is just insane - a lot.
The number of different job titles, professions can be estimated by some googling like "list of job titles in manufacturing" or similar - didn't have great success with finding some good place to link, this one isn't that bad https://www.bls.gov/ooh/ but requires specific IP addresses, others are less comprehensive. But I would guestimate like 500 job titles, but it just estimation on one site and do not think it comprehensive or detailed enough, so as you need few guys for each - junior, professional and senior in each category, as there is a job for all of them, so you need management as well. yes, some data scientists may be less relevant in the context of recreating steam era tech, but hey, where all the drawings come from, and how do you adjust them to your current need and maintain your original database.
But current specialties are not only ones such situation may require - in case of bootstrapping the technologies - it requires skills and expertise from past of the development of those technologies. working with vacuum tubes - is almost a lost art, even if it was common knowledge not so long ago, but today same TTL voltage or die. How cloth was made back in the days of a primitive era - only roleplayers know.
if your guys have to work, they do not have that much time to teach, not speaking about that doing and teaching are not the same activities - some can, some can't combine both.
So if we consider it all more detailed and the number of people required easy can swell from 100 to 10000 of geniuses people with mixed skills and education, maybe not geniuses but 120+ for sure. But next-generation won't be all 120+, genetics, sperm and eggs do not care about your needs. So to keep the presence of those smart people in sufficient quantities, do multiply it by another 10-20.
- if we skip cloning and artificial wombs, if not then some reduction coefficient can be squeezed from that.
And this way, if one digs it then it can be seen that 100 people is just not a starter no matter how you spin it if your goal is returning back to the technical capacity of your home. if primitive life is the goal then they all have survival training - all they need, and few thousand years later if they are lucky and do not die out and sail smoothly through the times they can get to our current time, do not forget your new technobible testament, however.
if they do have a technology to bootstrap technology
Then things are somewhat simpler. But then they do not have a situation of spare parts running out, nor do they need them as some big stock of them - what do they need is energy and resources materials elements. A broken part is not a broken part but an ore of elements they need to produce a perfectly working new part - so recycling is a king.
They need some system of equipment which can produce parts and components for each other. robust database - which keeps all the technological howtos, described in terms of that system and its automated capacities, and related information to resource gathering which fits the planet and equipment required for produce the equipment which does the resource gathering.
They need a lot of automatic - for servicing, for expansion, for transportation etc.
Steady energy source, a lot a lot of energy which is ready on demand and energy management systems to not run out of energy at critical moments of production. Fusion would be nice, D-fusion - then they will have no problems at any place suitable for humans. Energy from space also can be a good thing, SBSP, as they come from it then why not - it would help them a lot, and isn't that hard to establish if you already in space and all the technological capacities at hand.
Then the function of those guys is the management of the system, it still may require a good amount of brain cells and a wide variety of skills and knowledge.
Number still may be too low, but maybe, and for sure they also have to have automatic teaching systems, external means to transfer and imprint the required amounts of knowledge to next generations. This will be a very rigid system - no occupation changes allowed - for next generations it is, as you will need to squeeze it all from some 80 points individual, the same way as from a 100 or 120 guys and place them to proper occupations in that management system, you can't waste any human-like brain power and it still does not prevent the chances for them to drive that technological ship into the ground, because of lack of understanding what they do. if 100 people are enough to manage the system then again it needs more people or other ways to increase the numbers to guarantee to have the same quality of management in the future.
if we imagine some good AI assistance then 100 is enough and even one guy is sufficient. And then it less about people numbers but more about the social stability of the system which will define its future development and story.