Elesh Norn, Mother of Machines

Similarly, a permanent entering the battlefield at the same time as Elesh Norn (still including Elesh Norn) won't cause abilities of permanents an opponent controls to trigger.

If a triggered ability is linked to a second ability, additional instances of that triggered ability are also linked to that second ability. If the second ability refers to "the exiled card," it refers to all cards exiled by instances of the triggered ability.

In a Two-Headed Giant game, Elesh Norn does not affect triggered abilities of permanents your teammate controls.

In some cases involving linked abilities, an ability requires information about "the exiled card." When this happens, the ability gets multiple answers. If these answers are being used to determine the value of a variable, the sum is used. For example, if Elite Arcanist's enters-the-battlefield ability triggers twice, two cards are exiled. The value of X in the activation cost of Elite Arcanist's other ability is the sum of the two cards' mana values. As the ability resolves, you create copies of both cards and can cast none, one, or both of the copies in any order.

If a permanent entering the battlefield at the same time as Elesh Norn (including Elesh Norn itself) causes a triggered ability of a permanent you control to trigger, that ability triggers an additional time.

If you somehow control two Elesh Norns, a permanent entering the battlefield causes abilities to trigger three times, not four. A third Elesh Norn causes abilities to trigger four times, a fourth causes abilities to trigger five times, and so on.