I didn’t find this information anywhere. One user in Steam forums wrote he saw 9 spinning hammers. Skill description doesn’t mention any limits, so I take it - the higher % of block chance and higher reduced shield recovery should give me greater chance of having multiply hammers spinning around my characters. Is there a limit of maximum active hammers at once, besides individual hammer’s duration? Just being curious…
Limited solely by the duration of the hammers and the cooldown of the proc (which isn’t especially high).