Granted, it is difficult to know exactly, but it seems that knowledge was passed down in a master-pupil way, often based on personal connections. For example, in Lucian's The Dream he described how, as an adolescent, he was given to his uncle to be trained as a stonemason:

When my childhood was over, and I had just left school, my father called a council to decide upon my profession. Most of his friends considered that the life of culture was very exacting in toil, time, and money: a life only for fortune's favourites; whereas our resources were quite narrow, and urgently called for relief. If I were to take up some ordinary handicraft, I should be making my own living straight off, instead of eating my father's meat at my age; and before long my earnings would be a welcome contribution.

So the next step was to select the most satisfactory of the handicrafts; it must be one quite easy to acquire, respectable, inexpensive as regards plant, and fairly profitable. Various suggestions were made, according to the taste and knowledge of the councillors; but my father turned to my mother's brother, supposed to be an excellent statuary, and said to him: 'With you here, it would be a sin to prefer any other craft; take the lad, regard him as your charge, teach him to handle, match, and grave your marble; he will do well enough; you know he has the ability.' This he had inferred from certain tricks I used to play with wax. When I got out of school, I used to scrape off the wax from my tablets and work it into cows, horses, or even men and women, and he thought I did it creditably; my masters used to cane me for it, but on this occasion it was taken as evidence of a natural faculty, and my modelling gave them good hopes of my picking up the art quickly.

As soon as it seemed convenient for me to begin, I was handed over to my uncle, and by no means reluctantly; I thought I should find it amusing, and be in a position to impress my companions; they should see me chiselling gods and making little images for myself and my favourites. The usual first experience of beginners followed: my uncle gave me a chisel, and told me to give a gentle touch to a plaque lying on the bench: 'Well begun is half done,' said he, not very originally. In my inexperience I brought down the tool too hard, and the plaque broke; he flew into a rage, picked up a stick which lay handy, and gave me an introduction to art which might have been gentler and more encouraging; so I paid my footing with tears.

Lucian's can arguably be described as something like "middle class", as he was able to go to school to learn reading and writing. This, incidentally, is a fascinating depiction of the sort of status skilled laborers could attain. As for how many of them there were, that is unknowable, but it was enough to fill every European museum to the bursting with naked people made out of marble even after 2000 years.

I would like to add an additional question related to the sculpture linked specifically. My understanding is that it is widely regarded as one of the masterpieces of the era. Following links on the topic I noticed most criticisms of the piece all seemed to be from French critics of the 17th and 18th century.. Would there have been a reason behind French critics more harshly commenting than other nationalities? (Or am I just assuming the critical imbalance based on too small of a sample size of comments) thanks!