That would be 5. There are only 5 Legendary Polearms in the entire game.
I usually have my Demon Hunter craft 20 Sovereign Polearms and upgrade them in the Cube. I end up with at least 1 of all 5 Legendary Polearms with 1 to 3 of them being Bovine Bardiches.
I did not believe you, so I put it to the test. I created a Level 1 Crusader, had her craft 20 Sovereign Polearms and upgraded them to Legendary.
Instead of creating a male Barbarian to craft items for Kormac I’ll create a male Crusader instead. (In future Seasons). I’ll let him craft for my Templar and get my DH some Bovines.
I know there’s one Class that cannot upgrade to Bovine Bardiches, but I don’t remember which one that is. (Probably the Barbarian).
So, thank you for the tip. It’s going to make a significant change in my future D3 gaming routine. 
Now… if you’ll excuse me… I have to get back to my Crow Sandwich. 