If you control two Panharmonicons do your enter the battlefield triggers happen twice or three times?

Each Panharmonicon creates a replacement effect that causes a qualifying enter-the-battlefield (ETB) trigger to trigger an additional time. When you control two Panharmonicons, each one independently sees the ETB event and each adds one extra trigger. This means the original trigger fires once, plus one extra from the first Panharmonicon, plus one extra from the second, for a grand total of three triggers.

This works because Panharmonicon's effect is not a replacement effect that modifies the number of times something happens on a one-time basis — rather, each copy independently checks and doubles. Under CR 603.1 and the layering of triggered abilities, each Panharmonicon's effect stacks additively. Two Panharmonicons effectively give you 'trigger, trigger again, trigger again.'

Note that this applies only to artifact and creature ETB triggers, which is Panharmonicon's specific condition. Triggers from permanents that are not artifacts or creatures are unaffected.

Example: You control two Panharmonicons and cast Solemn Simulacrum (an artifact creature). Its ETB trigger would normally fire once (search for a basic land). With two Panharmonicons, it fires three times — you search for a basic land three separate times, putting three lands onto the battlefield tapped.

Reference CR 603.1 (triggered abilities trigger whenever their trigger event occurs) and CR 603.2 (each instance of a triggered ability is independent), which together explain why each Panharmonicon contributes its own additional trigger separately.