They had done a ton of market research and had to be pretty sure of their market estimates or they wouldn't have spent the money to design the bike in the first place.
It is not always like that. Sometimes they have designed and produced bikes that they knew from the start wouldn't be big sellers, like the turbos or the NR750. They did it to show their potential as bike manufacturers.
Who knows what crossed their minds at the time. The CB750 was born to tell the world that Honda was going into the big bike market. They knew the product was good but the customer welcome can seldom be anticipated. It makes sense to make a "batch" of bikes and, depending on the public's response, go on with the more expensive production method once the bike was already accepted.
As far as I know, no one had ever sold big bikes in the kind of volume that the CB750 was sold at. So it would be hard to anticipate what to expect. Even if market analysis said this would be the biggest hit ever, what does that mean? Outselling Norton, or Triumph, or BMW? Never big volumes.
The CB750 redefined big bike sales volume.
