Nope, multiple different cards stack but the same card doesn’t stack. However, the card’s effect works even when they’re not the leader.
For example; with the orc you can have 200 grunts, a tavern keeper, a vicious sergeant and a turtleshell sergeant (trolls). Regardless of which one is the leader, the tavern keeper +4 and v sergeant +3 will add their bonusses and the turtleshell will also add his +6 bonus when they’re in swamps but you’ll lose the v sergeant’s +3 bonus because that one only works in deserts. If 50 humans are added, the tavern keeper’s +4 bonus will also apply to them (because all) as will both other leaders when in their respective areas apply. However the general’s +5 will not buff to the orcs but will still buff the humans regardless,