To get back to your original question, I have looked through my resources and I cannot find a good solid number of workmen for 15th century Italian armouring workshops. However, we do have more details about the number of people working at other kinds of workshops, and this can help provide a kind of 'range' of people that might work at an armoury.

First, some background. When we are talking about the Italian armouring industry in the 15th century, we're mostly talking about Lombardy - the part of central Northern Italy in the Po valley, where Milan is. In particular, we are talking about the cities of Milan, perhaps the greatest armouring center in Europe, and Brescia, its upstart rival. Rome and Florence, while important in the other arts, were not major armouring centers. There are a variety of reasons for this - neither was as large as Milan, and Milan had access to trade routes across the alps as well as to the Genoese and Venetian trade routes. Moreover, Milan (and Brescia!) was near a good source of iron ore, like all of the truly great European armouring centers of the late Middle Ages. Rome and Florence would have a few armourers, but in neither city were there enough to form an export business; indeed some armourers in Rome in the early 16th century were branches of Milanese armouring families (as was also the case in Naples and elsewhere in Europe).

I talk more about what makes the organization of Lombard armourers special in the answers linked above. But for our purposes here the important thing to remember is that Lombard armourers organized themselves around a system of contracts, rather than a master/journeyman/apprentice guild system. These contracts could take on a variety of forms - in some cases, it was a fairly straightforward agreement to work for another armourer for a term (typically two years), often making a particular piece of armour, like a helmet. The final armour would then be assembled by a senior armourer, the traversator This work might be done by the subcontractor in the senior partner's shop, or it might be done in the subcontractor's own workshop (which way this worked has implications for workshop size, obviously; a workshop with many subcontractors working all in one roof would be quite large, while a workshop that outsourced its piece-work to other workshops might simply be a traversator and his assistants). There were also more sophisticated arrangements between armourers and between armourers and merchants - in some cases, one party would provide the money and materials and the other would provide all the labor; the party providing the money would then receive 2/3s of the profits (and in this the 15th century armouring industry starts to look like capitalism). There were other, more complicated partnerships, all drawn out in contracts. The upshot of this is that Milanese armourers might work together under one roof, but they also had a lot of ways to create economies of scale without combining workshops.

So, how big did these workshops get? I have some indirect evidence from Italy, and some direct evidence from elsewhere. First, the indirect evidence from elsewhere. In the early 16th century Hans Seusenhofer was engaged by Maximilian I to make an armoury for the Dukes of Austria in Innsbruck. His armoury had a dozen people working in it - journeyman hammerers, apprentices, and specialized polishers, with Seusenhofer acting as master armourer. This gives you some idea of how large a well-equipped, well funded armoury might be; Seusenhofer both made bespoke armour for the Emperor and the nobility of the Empire and made mass-produced armour for the Emperor's soldiers. On the other end of the spectrum, small armouries throughout Europe could be as small as a master with a single journeyman and an apprentice; in Milan an armourer like this would almost certainly be a subcontractor, even if he retained his own shop. My evidence for Milanese armourers is less direct. We know that in the 16th century three of the Negroli brothers lived and worked together in the family house/workshop, and that when this property was divided it featured dozens of stakes, anvils and hammers, and was worth thousands of Scudi - obviously a large operation. Given that the armoury featured 3 masters working under one roof, each of whome probably had some assistants, that is at least a half-dozen people (probably more) working in an armoury that made only 'boutique' orders for the wealthiest patrons.

As to how armouries for 'munition' armour were organized versus armouries for 'bespoke' armour, this is hard to say based on my reading. We know that in some places like Nuremberg 'munition' armour was made in small workshops who were 'hired out' to make portions of a large order of armours. We also know that large armouries, like Seusenhofer's, could make both mass-produced munition armour and bespoke armour for Emperors. So there is not an immediately apparent close relationship between workshop size and armour quality, since so much work is 'hired out'. However, I will let you know if I find anything else in further reading.

Sources:

Nickolas Dupras - Armourers and their Workshops

Alan Williams - The Knight and the Blast Furnace

Phyrr and Leydi - Heroic Armour of the Italian Renaissance

Matthias Pffafenbach - Armourers

To answer your follow-up questions first:

• Regarding the source of metal plates and the general workflow, steel/iron plate of the 15th and 16th century was not, generally, produced 'in house'. I talk about this and the steps of making armour (from ore to harness, and all the hands it passed through), here.

• I address the quality differences between 'grades' of armour here.