Merely a reasonable inference, Les. The number of broadside printers was limited and they frequently stole songs from one another. Surely the shop owners would not hire more songwriters than they could afford to pay, and anyone who could write one suficiently good broadside text was likely capable of writing another and another and another. Some of the likeliest writers might be the printers and publishers themselves, not to mention their spouses.

My estimate of "several dozen" (which could mean 100 or 150 or more) was intended to provoke, not stifle, thought about the matter. Our usual assumption seems to be that "common folk" all over the British Isles were constantly creating new texts; perhaps they were, but only a finite number of original songs made between, say, 1800 and 1900 became truly "traditional." A very large number of independently
inspired amateurs writing one or two texts each would not, I think, be necessary to account for all of the superior texts. (And I'm speaking only of texts, not tunes.)

Part (and only part) of the reason that the style of the broadsides is so conventional may be that not many people were behind them.

I'm not claiming any special wisdom here, just raising some possibilities that need to be addressed, if they have not been already.