C1880 'The Cruise of The Calabar' on sheet music has the line 'There are but 40 verses so it won't detain yiz long'. Looking at a few versions on broadsides and from oral tradition 40 is also the number given in all of them. Of course it doesn't actually have 40 verses.

Quite possibly some wag just chose that number 47 at random then it caught on and became a cliché.

On the other hand some phd student probably did a survey in the ballad clubs and found that was the mean point in a ballad when most people fell asleep.

As for Tam-lin, Scott's 1833 version has 56 and Peter Buchan not to be outdone has 59. Hmmm! Wonder how many Bert Lloyd had.